Visual long-term memory for spatial frequency?

It has been suggested that a visual long-term memory based on a sensory representation of the stimulus accounts for discrimination performance when the reference and the test stimuli are separated in time. Decision processes involved in setting response criteria, however, may also contribute to discrimination performance. In the present study, it is shown that under proper control, spatial frequency discrimination thresholds from a group of observers, each performing on a single trial, are significantly higher for a 2-h than for a 5-sec retention interval, whereas thresholds from individual observers performing in repeated trials with a 2-h retention interval are considerably lower. The results suggest that discrimination performance may depend on the retention of task-relevant information, such as a response criterion, rather than on visual memory of the stimulus. It is concluded that it is risky to postulate ahigh-fidelity long-term visual memory for spatial frequency on the basis of psychophysical group discrimination thresholds.     

Perfect visual short-term memory for periodic patterns

Abstract

The short-term memory for spatial frequency information was assessed by measuring the spatial frequency discrimination thresholds for briefly flashed luminance gratings as a function of the time interval between the test and reference gratings, using a computer-controlled two-interval forced-choice procedure. Discrimination thresholds were stable for interstimulus intervals in the range 1–30 sec under all conditions tested. At low contrasts, short exposure times and low spatial frequencies discrimination thresholds increased, but no interactions between stimulus parameters affecting thresholds and interstimulus interval were observed. It is concluded that factors limiting spatial discrimination are associated with the sensory coding stage. Spatial discrimination and visual memory may be based on a common representation, which is perfectly retained in short-term memory. Visual half-field tests revealed no hemispheric differences in the processing and retention of spatial frequency information.     

High-fidelity perceptual long-term memory revisited--and confirmed

Experiments on short-term perceptual memory for elemental visual attributes, such as contrast, motion, orientation, and spatial frequency, have relied on a delayed discrimination technique in which the subject compares two stimuli presented at different points in time and memory is indexed by discrimination thresholds measured for the different time intervals between reference and test. In a variant of this procedure, used in experiments on long-term memory, the presentation of a single reference is followed by a memory test that combines two-alternative forced-choice decisions with the method of constant stimuli. With this procedure, it is impossible to distinguish the effects of criterion-setting processes and memory on performance, but this confound can be eliminated by testing many subjects and having each subject make a single decision. The resulting "group thresholds" are stable across time intervals of 24 hr, confirming previous findings of high-fidelity storage in the long-term memory range.     

The stereoscopic anisotropy: individual differences and underlying mechanisms

Observers are more sensitive to variations in the depth of stereoscopic surfaces in a vertical than in a horizontal direction; however, there are large individual differences in this anisotropy. The authors measured discrimination thresholds for surfaces slanted about a vertical axis or inclined about a horizontal axis for 50 observers. Orientation and spatial frequency discrimination thresholds were also measured. For most observers, thresholds were lower for inclination than for slant and lower for orientation than for spatial frequency. There was a positive correlation between the 2 anisotropies, resulting from positive correlations between (a) orientation and inclination thresholds and (b) spatial frequency and slant thresholds. These results support the notion that surface inclination and slant perception is in part limited by the sensitivity of orientation and spatial frequency mechanisms.     

Detection of amplitude modulation and frequency modulation in tactual gratings: a critical bandwidth for active touch

Since most natural surfaces are complex and vary in amplitude and spatial frequency, it might be interesting to consider gratings not in the spatial domain, but in the spatial-frequency domain. Detection thresholds for amplitude modulation (AM) and frequency modulation (FM) in sinusoidal gratings were measured for seven participants. Participants moved their fingers actively across the gratings. Although the two types of modulation are quite different in the spatial domain, they have many features in common in the frequency domain. In previous research (Nefs et al 2001 Perception 30 1263-1274) we measured the discrimination thresholds for amplitude and frequency for sinusoidal gratings. We hypothesised then that these thresholds could be used to predict the discriminability of other types of gratings. In the present study, we did indeed find that the FM and AM detection thresholds can be understood quite well by these discrimination thresholds. The results indicate that the tactual system contains parallel psychophysical channels that filter and integrate the power of stimuli within critical bands. With these results, we are also able to calculate the critical bandwidth for active dynamic touch. We estimated the critical bandwidth surrounding the spatial frequency of 2 cycles cm(-1) to be about 125% of that spatial frequency. This value for the critical band for spatial frequency is incompatible with previous findings for temporal frequencies in vibrotactile research. This indicates that dynamic spatial-frequency discrimination is not likely to be done by temporal frequency.     

The influence of spatial pattern on visual short-term memory for contrast

Several psychophysical studies of visual short-term memory (VSTM) have shown high-fidelity storage capacity for many properties of visual stimuli. On judgments of the spatial frequency of gratings, for example, discrimination performance does not decrease significantly, even for memory intervals of up to 30 s. For other properties, such as stimulus orientation and contrast, however, such “perfect storage” behavior is not found, although the reasons for this difference remain unresolved. Here, we report two experiments in which we investigated the nature of the representation of stimulus contrast in VSTM using spatially complex, two-dimensional random-noise stimuli. We addressed whether information about contrast per se is retained during the memory interval by using a test stimulus with the same spatial structure but either the same or the opposite local contrast polarity, with respect to the comparison (i.e., remembered) stimulus. We found that discrimination thresholds got steadily worse with increasing duration of the memory interval. Furthermore, performance was better when the test and comparison stimuli had the same local contrast polarity than when they were contrast-reversed. Finally, when a noise mask was introduced during the memory interval, its disruptive effect was maximal when the spatial configuration of its constituent elements was uncorrelated with those of the comparison and test stimuli. These results suggest that VSTM for contrast is closely tied to the spatial configuration of stimuli and is not transformed into a more abstract representation.     

Spatial-frequency discrimination, brain lateralisation, and acute intake of alcohol

The effect of alcohol (breath-alcohol level of 0.1%) on perceptual discrimination of low (1.5 cycles deg-1) and high (8 cycles deg-1) spatial frequencies in the left and right visual field was measured in eighteen right-handed males, in a double-blind, balanced placebo design. Discrimination thresholds for briefly (180 ms) presented sinusoidal gratings were determined by two-alternative forced-choice judgments with four interleaving psychophysical staircases providing random trial-to-trial variation of reference spatial frequency and visual field, in addition to a random (+/- 10%) jitter of reference spatial frequency. Alcohol produced overall higher discrimination thresholds but did not alter the visual-field balance: no main effect of visual field was observed, but in both placebo and alcohol conditions spatial frequency interacted with visual field in the direction predicted by the spatial-frequency hypothesis of hemispheric asymmetry in visual-information processing, with left-visual-field/right-hemisphere superiority in discrimination of low spatial frequencies and right-visual-field/left-hemisphere superiority in discrimination of high spatial frequencies.     

Learning to discriminate simple sinusoidal gratings is task specific

Learning transfer effects of a modified spatial frequency discrimination task with simple sinusoidal gratings on various untrained test tasks, testing performance in relative position, local width and global size discrimination, have been investigated. Six subjects were exposed to grating stimuli of varying spatial frequency and size but constant relative position while they had to respond only to spatial frequency. In the course of 7 consecutive days all subjects showed significant reduction of spatial frequency discrimination thresholds. Comparison of discrimination thresholds for the untrained test tasks, taken before and after the learning epoch, reveals complete learning transfer to spatial frequency discrimination with gratings of constant size and variable relative position, but complete lack of transfer when grating spatial frequency is shifted about one octave lower. Further, there is improvement of relative position discrimination and width discrimination of single luminance bars, which is not very specific for the learnt spatial frequency. Although size variation was part of the learning procedure, discrimination of global stimulus size did not improve. Generally, the observed scheme of learning transfer reveals that there is learning only for stimulus attributes that are behaviorally relevant in the learning task. The differential scheme of improvement and code usage in the test tasks strongly indicates involvement of higher stages capable of independent access to different coding domains, as well as attentionally guided attribute selection and suppression. Supported by other recent findings, it is suggested that discrimination learning can be understood as a higher level process of gradual refinement of code selection out of a rich code base provided by lower level stages.     

The precision of velocity discrimination across spatial frequency

The precision of velocity coding for moving stimuli of different spatial frequencies was assessed by measuring velocity discrimination thresholds for a 1-c/deg grating paired with a grating whose spatial frequency ranged from 0.25 to 4 c/deg and for grating pairs of the same spatial frequency (0.25, 1, and 4 c/deg). The gratings always moved upward, with velocities ranging from 0.5 to 16 deg/sec, Velocity discrimination was as precise for stimuli that varied in spatial frequency by: ±2 octaves (0.25 vs. 1 c/deg and 4 vs. 1 c/deg) as for stimuli of the same spatial frequency, for specific ranges of velocity that depended on the spatial and, therefore, the temporal frequencies of the stimuli. Compared with a 1-c/deg grating, the perceived velocity of 4-c/deg gratings was about 1.3 times faster and that of 0.25-c/deg gratings was about 1.3 times slower. Although these perceived velocity biases imply variation of velocity-signal processing among spatial frequency channels, the discrimination results indicate that the motion-sensing system can compare signals across different spatial frequency channels to make fine velocity discrimination within appropriate temporal frequency limits.     

Retention and disruption of motion information in visual short-term memory.

Velocity discrimination thresholds for drifting luminance gratings were measured as a function of the time interval between test and reference gratings, using a two-interval, forced-choice procedure. Discrimination thresholds, expressed as Weber fractions (delta V/V), were independent of interstimulus intervals (ISIs) ranging from 1-30 s, demonstrating perfect short-term retention of velocity information. When a third grating was briefly presented halfway through a 10-s ISI, memory masking was observed. Discrimination thresholds in memory masking were unaffected by maskers of the same velocity but increased by 100% when test and masker velocity differed by a factor of 2. The results are interpreted with reference to a model where the short-term memory for simple stimulus attributes is assumed to be organized in terms of arrays of memory stores linked in a lateral inhibitory network.     

The effects of aging on visual memory: evidence for functional reorganization of cortical networks

Recent evidence suggests that the mature human brain is capable of substantial functional reorganization following injury. The fact that the brain retains a great deal of plasticity raises the possibility that cortical reorganization may occur during normal aging. We examined this issue by using positron emission tomography (PET) to measure the brain activity associated with short-term memory for simple visual attributes in young and old observers. A two-interval forced choice procedure was used to measure spatial frequency discrimination thresholds for sine wave gratings presented at different inter-stimulus intervals (ISI). Memory load was manipulated by varying the duration of the ISI and by presenting an irrelevant masking stimulus in the middle of the ISI. Old and young observers performed the experiment equally well. However, the neural systems correlated with good performance differed for the two age groups. The results support the hypothesis that the functional networks that underlie visual memory undergo reorganization during aging.     

Perceptual asymmetries are preserved in short-term memory tasks

Visual performance is heterogeneous at isoeccentric locations; it is better on the horizontal than on the vertical meridian and worse at the upper than at the lower region of the vertical meridian (Carrasco, Talgar, & Cameron, 2001; Talgar & Carrasco, 2002). It is unknown whether these performance inhomogeneities are also present in spatial frequency tasks and whether asymmetries present during encoding of visual information also emerge in visual short-term memory (VSTM) tasks. Here, we investigated the similarity of the perceptual and VSTM tasks in spatial frequency discrimination (Experiments 1 and 2) and perceived spatial frequency (Experiments 3 and 4). We found that (1) performance in both simultaneous (perceptual) and delayed (VSTM) spatial frequency discrimination tasks varies as a function of location; it is better along the horizontal than along the vertical meridian; and (2) perceived spatial frequency in both tasks is higher along the horizontal than along the vertical meridian. These results suggest that perceived spatial frequency may mediate performance differences in VSTM tasks across the visual field, implying that the quality with which we encode information affects VSTM.     

Amplitude and spatial-period discrimination in sinusoidal gratings by dynamic touch

We measured discrimination thresholds for sinusoidal gratings using active dynamic touch. In the first experiment we measured the thresholds for amplitude discrimination as a function of amplitude and spatial period. Thresholds ranged between 10.8%, and 15.8% of the standard amplitude. We showed that amplitude differences as small as 2 microm can be detected. We found that Weber fractions for amplitude discrimination are constant over a range of amplitudes, but are influenced by the spatial period of the grating; discrimination improved with increasing spatial period. In the second experiment we determined the thresholds for spatial-period discrimination. We used the same design as in the first experiment. Weber fractions ranged from 6.4% to 11.8%. Amplitude was found to have no effect on the Weber fractions for spatial-period discrimination. However, the spatial period had an effect on the Weber fractions: larger spatial periods yielded lower Weber fractions.     

Spatial frequencies in short-term memory for faces: A test of three frequency-dependent hypotheses

Spatial frequency manipulations have been shown to have significant utility in ascertaining the various types of information that might be important in object identification and recognition tasks. This utility suggests that, given some mapping between ranges of spatial frequencies and different types of psychological information, it should be possible to examine the roles of these different types of psychological information by way of spatial frequency manipulations. One potential problem, however, is that there is no well-specified, unambiguous mapping between the distinctions in the frequency domain and the distinctions in the informational domain. Three experiments provide tests of three general hypotheses regarding the ways in which different spatial frequencies might map to different information in facial perception and memory tasks: (1) the low-frequency dominance hypothesis, which proposes that low-frequency information should be superior (to high-frequency information) as a cue to perception and memory; (2) the distinct informational roles hypothesis, which holds that high spatial frequencies should carry featural information while low spatial frequencies should carry information about the configuration of those features; and (3) the task-dependent information hypothesis, which suggests that high-frequency information should be best suited to discrimination tasks while low-frequency information should be best suited for recognition tasks. Results generally contradict the first two of these hypotheses, while providing support for the third. Implications with regard to the various issues related to the mapping between spatial frequencies and the informational content of faces, as well as the need to consider important interactions among perceptual and memory processes, are discussed.     

Detection and discrimination of subjective contours defined by offset gratings

Three experiments were conducted to refine our understanding of the mechanisms that encode subjective contours. In Experiment 1, discrimination thresholds (stimulus onset asynchronies [SOAs] yielding 81% correct) were measured in a backward masking paradigm for subjective contours defined by offset gratings. For large apertures, thresholds increased as carrier frequency increased. For the smallest aperture, thresholds were a U-shaped function of carrier frequency. Experiment 2 showed that these threshold results were generally consistent with the rated strength of the subjective contours. Experiment 3 showed that detection thresholds (SOAs yielding 81% correct) again increased with carrier spatial frequency, increased for obliquely oriented carriers, and, for a particular frequency and orientation of the carrier, were lower when the subjective contour was orthogonal to the carrier. All of these results are well explained by a two-stage process in which a second-layer filter integrates the responses of end-stopped mechanisms to the terminators defining the subjective contour. In the model, the end-stopped mechanisms have low-pass sensitivity to carrier spatial frequency, and the sizes of the second-layer filters are proportional to the scale of the end-stopped mechanisms from which they draw their input.     

Working memory resources are shared across sensory modalities

A common assumption in the working memory literature is that the visual and auditory modalities have separate and independent memory stores. Recent evidence on visual working memory has suggested that resources are shared between representations, and that the precision of representations sets the limit for memory performance. We tested whether memory resources are also shared across sensory modalities. Memory precision for two visual (spatial frequency and orientation) and two auditory (pitch and tone duration) features was measured separately for each feature and for all possible feature combinations. Thus, only the memory load was varied, from one to four features, while keeping the stimuli similar. In Experiment 1, two gratings and two tones—both containing two varying features—were presented simultaneously. In Experiment 2, two gratings and two tones—each containing only one varying feature—were presented sequentially. The memory precision (delayed discrimination threshold) for a single feature was close to the perceptual threshold. However, as the number of features to be remembered was increased, the discrimination thresholds increased more than twofold. Importantly, the decrease in memory precision did not depend on the modality of the other feature(s), or on whether the features were in the same or in separate objects. Hence, simultaneously storing one visual and one auditory feature had an effect on memory precision equal to those of simultaneously storing two visual or two auditory features. The results show that working memory is limited by the precision of the stored representations, and that working memory can be described as a resource pool that is shared across modalities.     

Elaborative rehearsal of nontemporal information interferes with temporal processing of durations in the range of seconds but not milliseconds

The distinct timing hypothesis suggests a sensory mechanism for processing of durations in the range of milliseconds and a cognitively controlled mechanism for processing of longer durations. To test this hypothesis, we employed a dual-task approach to investigate the effects of maintenance and elaborative rehearsal on temporal processing of brief and long durations. Unlike mere maintenance rehearsal, elaborative rehearsal as a secondary task involved transfer of information from working to long-term memory and elaboration of information to enhance storage in long-term memory. Duration discrimination of brief intervals was not affected by a secondary cognitive task that required either maintenance or elaborative rehearsal. Concurrent elaborative rehearsal, however, impaired discrimination of longer durations as compared to maintenance rehearsal and a control condition with no secondary task. These findings endorse the distinct timing hypothesis and are in line with the notion that executive functions, such as continuous memory updating and active transfer of information into long-term memory interfere with temporal processing of durations in the second, but not in the millisecond range.     

Prolonged development of auditory skills: A role for perceptual anchoring?

Many auditory skills continue to develop beyond infancy and even into adolescence, but the factors underlying this prolonged development remain poorly understood. Of interest here is the contribution of on-line statistical learning of stimulus repetitions (anchoring) to the development of auditory spectral and temporal discrimination, as well as the potential contributions of auditory attention and working memory. Children, aged 6–13 years, as well as adults (age range: 21–33 years) were tested on auditory frequency and duration discrimination. Each type of discrimination was measured in two conditions (XAB and XXXAB) designed to afford different levels of anchoring by varying the number of repetitions of a standard stimulus (X) prior to the presentation of the test tone (A or B) in each trial. Auditory attention and working memory were also assessed. Whereas duration and frequency discrimination in either condition did not reach adult level prior to 11 years of age, the magnitude of the anchoring effect was similar across ages. These data suggest that perceptual anchoring matures prior to the attainment of adult-like discrimination thresholds. Likewise, neither attention nor working memory could account for the observed developmental trajectories. That auditory discrimination and anchoring follow dissociable developmental trajectories suggests that different factors might contribute to the development of each. We therefore conclude that although anchoring might be necessary for attaining good auditory discrimination, it does not account for the prolonged development of auditory frequency and duration discrimination in school-aged children.     

The effect of marker frequency disparity on the discrimination of gap duration in monkeys

Duration-discrimination thresholds of the silent interval (gap) between two successive tones (markers) were measured in four Japanese monkeys. The task was serial discrimination, and monkeys were required to release the lever when the gap duration decreased from 200 ms. Monkeys successfully acquired the task, and gap thresholds of monkeys were revealed to be larger than previous data with human subjects. Gap thresholds were not affected by marker frequency when the two markers were identical in frequency, though the thresholds increased when large frequency differences existed between markers. The effect of marker frequency disparity on gap thresholds in monkeys is discussed in terms of the difficulty in integrating information from discrete frequency channels.     

The posterior parietal cortex and long-term memory representation of spatial information

The hypothesis to be explored in this chapter is based on the assumption that the posterior parietal cortex (PPC) is directly involved in representing a subset of the spatial features associated with spatial information processing and plays an important role in perceptual memory as well as long-term memory encoding, consolidation, and retrieval of spatial information. After presentation of the anatomical location of the PPC in rats, the nature of PPC representation based on single spatial features, binding of visual features associated with visual spatial attention, binding of object-place associations associated with acquisition and storage of associations where one of the elements is a spatial component, and binding of ideothetic and allothetic information in long-term memory is discussed. Additional evidence for a PPC role in mediation of spatial information in long-term storage is offered. Finally, the relationship between the PPC and the hippocampus from a systems and dynamic point view is presented.