The effect of warning interval on the electric phosphene and auditory thresholds

Using different warning signals and threshold stimuli, the thresholds, as determined by a method of limits, were found to rise monotonically as the interval between warning signal and threshold stimulus increased from I to 9 sec. It was found that the variability of the threshold did not increase as the threshold increased. Similar results were obtained for phosphene and auditory thresholds and with visual and auditory warnings; therefore the effect was considered to be central. Motokawa's finding of a minimum in the phosphene threshold 2 sec. after a flash of white light was not repeated. The rise in threshold was not obtained when the warning intervals were randomized and so seemed to depend on the use of fixed warning intervals. A model was developed relating threshold level to accuracy of anticipation of the end of the warning interval.     

Multi- and unisensory visual flash illusions

The role of stimulus structure in multisensory and unisensory interactions was examined. When a flash (17 ms) was accompanied by multiple tones (each 7 ms, SOA < or =100 ms) multiple flashes were reported, and this effect has been suggested to reflect the role of stimulus continuity in multisensory interactions. In experiments 1 and 2 we examined if stimulus continuity would affect concurrently presented stimuli. When a relatively longer flash (317 ms) was accompanied by multiple tones (each 7 ms), observers reported perceiving multiple flashes. In experiment 3 we tested whether a flash presented near fixation would induce an illusory flash further in the periphery. One flash (17 ms) presented 5 degrees below fixation was reported as multiple flashes if presented with two flashes (each 17 ms, SOA =100 ms) 2 degrees above fixation. The extent to which these data support a phenomenological continuity principle and whether this principle applies to unisensory perception is discussed.     

The differential effects of brief exposures and surrounding contours on direct and indirect tilt illusions

Four experiments in which logarithmic intervals between 25 and 1600 ms were used for stimulus duration in tests for the tilt illusion are reported. It is demonstrated that the direct and the indirect tilt illusions both increase in magnitude inversely with length of stimulus presentation. The data suggest that whereas the direct effect peaks with a value of about +7 degrees at the shortest flash duration used (25 ms), peak indirect effects (of about +2 degrees) do not occur at this duration. In addition, whereas direct effects level out after 100 ms stimulus exposure times, to the usual magnitude obtained with long presentations (about +2 degrees), indirect effects reach their standard magnitude (-0.5 degrees to -1.0 degrees) later, at exposures of about 400 ms. Even at very short flash durations, a luminance square frame surrounding the illusion display reduces the indirect effect by two thirds of its magnitude but has no effect at all on the direct effect. It is suggested that direct effects arise early in visual processing, in area V1, where there are transient mechanisms and where corruption of orientation analysis by the inducing grating would occur prior to later, extrastriate, global analysis of the surrounding peripheral frame. Indirect effects, on the other hand, may arise later, along the sustained parvocellular colour-form pathway, where more global processing occurs and susceptibility to surrounding fields might be expected.     

Backward and forward masking associated with saccadic eye movement

Visual masking effects on test flash thresholds were measured under real and simulated eye movement conditions to determine whether visual masking is primarily responsible for elevations in threshold that are sometimes associated with saccadic eye movements. Brief luminous flashes presented to the central retina before, during, and after saccades were masked by stimuli presented either pre- or postsaccadically. The amount and time course of masking were quantitatively dependent on stimulus parameters of intensity and temporal separation and were unaffected by eye movement parameters (amplitude, velocity, direction) as long as retinal stimulus conditions were constant. The duration of forward masking was longer than that of backward masking. When retinal conditions during saccades were mimicked while the eyes were held steady, masking interactions were identical to those obtained during real saccades. These results indicate that masking effects during saccades in ordinary environments are determined solely by the stimulus situation at the retina. Putative nonvisual, centrally originating saccadic suppression suggested by other authors is evidently not additive with visually determined masking during saccades.     

Two-flash thresholds as a function of flash luminance and area

Two-flash thresholds were obtained from three dark-adapted Ss by means of a two-interval, temporal forced-choice technique involving foveally fixated circular targets varying in luminance and area. The Ss were instructed to report the temporal position of the longer of two pairs of I-msec flashes. a comparison pair with an interflash interval of 1 msec, and a test pair with a varying interflasn interval. The Ss were informed about the accuracy of their responses after each response. For all three Ss the results replicated a previous finding that two-flash thresholds are a negatively accelerated function of flash luminance. but the function was shown to be dependent on area. a greater threshold change occurring at smaller areas. Two-flash thresholds were also found to be a decreasing function of stimulus area. with the greatest threshold change occurring at low luminances.     

Continuous measurement of visible persistence

In the synchrony judgment paradigm, observers judge whether a click precedes or follows the onset of a light flash and, on other trials, whether or not a click precedes light termination. The interclick interval defines the duration of visible persistence. An elaboration of this method consists of two phases: In Phase 1, the luminance of a reference stimulus is psychophysically matched to the peak brightness of the test flash. Five luminance values between .1 and 1.0 of the reference stimulus are used subsequently. In Phase 2, a random one of the five reference stimuli, a test flash, and a click are presented; the observer judges whether the click occurred before or after the brightness of test flash reached the reference value (on onset trials) or decayed below it (on termination trials). This method was validated on 3 subjects with test stimuli whose luminance rises and decays slowly in time, and then was used to trace out the precise subjective rise and decay (temporal brightness response function) of brief flashes.     

Orientation discrimination across the visual field: size estimates near contrast threshold

Performance in detection and discrimination tasks can often be made equal across the visual field through appropriate stimulus scaling. The parameter E2 is used to characterize the rate at which stimulus dimensions (e.g., size or contrast) must increase in order to achieve foveal levels of performance. We calculated both size and contrast E2 values for orientation discrimination using a spatial scaling procedure that involves measuring combination size and contrast thresholds for stimuli with constant size-to-contrast ratios. E2 values for size scaling were 5.77 degrees and 5.92 degrees. These values are three to four times larger than those recovered previously using similar stimuli at contrasts well above detection threshold (Sally & Gurnsey, 2003). E2 values for contrast scaling were 324.2 degrees and 44.3 degrees, indicating that for large stimuli little contrast scaling (.3% to 2.3% increase) was required in order to equate performance in the fovea and the largest eccentricity (10 degrees). A similar pattern of results was found using a spatial scaling method that involves measuring contrast thresholds for target identification as a function of size across eccentricities. We conclude that the size scaling for orientation discrimination at near-threshold stimulus contrasts is much larger than that required at suprathreshold contrasts. This may arise, at least in part, from contrast-dependent changes in mechanisms that subserve task performance.     

The spectral sensitivities of the middle- and long-wavelength cones: an extension of the two-colour threshold technique of W S Stiles

When a tiny centred test flash is presented on a small concentric background, the threshold rises with background radiance more quickly than Weber's law would predict. It is argued that under such conditions it is possible, by means of a test sensitivity method, to isolate either the M-cone or the L-cone types throughout the visible spectrum. As predicted, double-branched M- and L-cone tyr functions are found when the test flash and the field are of the same wavelength. From the independent vertical displacements of the two branches as test wavelength is varied, it is possible to derive spectral sensitivities that agree well with dichromatic sensitivities and K?nig fundamentals. The test sensitivities deviate from pi 4 at longer wavelengths and from pi 5 at shorter wavelengths.     

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.     

Segregation by color/luminance does not necessarily facilitate motion discrimination in the presence of motion distractors.

Under what circumstances is the common motion of a group of elements more easily perceived when the elements differ in color and/or luminance polarity from their surround? Croner and Albright (1997), using a conventional global motion paradigm, first showed that motion coherence thresholds fell when target and distractor elements were made different in color. However, in their paradigm, there was a cue in the static view of the stimulus as to which elements belonged to the target. Arguably, in order to determine whether the visual system automatically groups, or prefilters, the image into different color maps for motion processing, such static form cues should be eliminated. Using various arrangements of the global motion stimulus in which we eliminated all static form cues, we found that global motion thresholds were no better when target and distractors differed in color than when they were identical, except under certain circumstances in which subjects had prior knowledge of the specific target color. We conclude that, in the absence of either static form cues or the possibility of selective attention to the target color, features with similar colors/luminance-polarities are not automatically grouped for global motion analysis.     

Segregation by color/luminance does not necessarily facilitate motion discrimination in the presence of motion distractors

Under what circumstances is the common motion of a group of elements more easily perceived when the elements differ in color and/or luminance polarity from their surround? Croner and Albright (1997), using a conventional global motion paradigm, first showed that motion coherence thresholds fell when target and distractor elements were made different in color. However, in their paradigm, there was a cue in the static view of the stimulus as to which elements belonged to the target. Arguably, in order to determine whether the visual system automatically groups, or prefilters, the image into different color maps for motion processing, such static form cues should be eliminated. Using various arrangements of the global motion stimulus in which we eliminated all static form cues, we found that global motion thresholds were no better when target and distractors differed in color than when they were identical, except under certain circumstances in which subjects had prior knowledge of the specific target color. We conclude that, in the absence of either static form cues or the possibility of selective attention to the target color, features with similar colors/luminance-polarities are not automatically grouped for global motion analysis.     

The perception of identity by 6 1/2-month-old infants.

In Study 1, sixteen 6 1/2-month-olds were habituated to a Reversible stimulus (an upright face that could be perceived as an entirely different upright face when it was rotated 180 degrees) and to a Nonreversible stimulus (a face that could be perceived as upright in only one orientation). Following habituation for each type of stimulus, test trials paired the habituated face with a novel stimulus (an inversion of the same face). For both Reversible and Nonreversible stimuli, the physical difference between the old and new test stimuli was the same (a 180 degrees rotation); however, infants devoted more visual attention to the 180 degrees rotation only when it was a Reversible face, suggesting that the identity change was detected. Experiment 2 ruled out the explanation that infants might have failed to dishabituate to the inversion of the Nonreversible stimulus because they could not remember it. Results are interpreted as evidence that 6 1/2-month-old infants are not limited to face recognition based on similarity in pattern arrangement alone, but are capable of processing faces at a representational level.     

Anticipation of visual form independent of knowing where the form will occur

We investigated how selective preparation for specific forms is affected by concurrent preknowledge of location when upcoming visual stimuli are anticipated. In three experiments, participants performed a two-choice response time (RT) task in which they discriminated between standard upright and rotated alphanumeric characters while fixating a central fixation cross. In different conditions, we gave the participants preknowledge of only form, only location, both location and form, or neither location nor form. We found main effects of both preknowledge of form and preknowledge of location, with significantly lower RTs when preknowledge was present than when it was absent. Our main finding was that the two factors had additive effects on RTs. A strong interaction between the two factors, such that preknowledge of form had little or no effect without preknowledge of location, would have supported the hypothesis that form anticipation relies on depictive, perception-like activations in topographically organized parts of the visual cortex. The results provided no support for this hypothesis. On the other hand, by an additive-factors logic Sternberg (Sternberg, Acta Psychologica 30:276?C315, 1969), the additivity of our effects suggested that preknowledge of form and location, respectively, affected two functionally independent, serial stages of processing. We suggest that the two stages were, first, direction of attention to the stimulus location and, subsequently, discrimination between upright and rotated stimuli. Presumably, preknowledge of location advanced the point in time at which attention was directed at the stimulus location, whereas preknowledge of form reduced the time subsequently taken for stimulus discrimination.     

Saccades reveal that allocentric coding of the moving object causes mislocalization in the flash-lag effect

The flash-lag effect is a visual misperception of a position of a flash relative to that of a moving object: Even when both are at the same position, the flash is reported to lag behind the moving object. In the present study, the flash-lag effect was investigated with eye-movement measurements: Subjects were required to saccade to either the flash or the moving object. The results showed that saccades to the flash were precise, whereas saccades to the moving object showed an offset in the direction of motion. A further experiment revealed that this offset in the saccades to the moving object was eliminated when the whole background flashed. This result indicates that saccadic offsets to the moving stimulus critically depend on the spatially distinctive flash in the vicinity of the moving object. The results are incompatible with current theoretical explanations of the flash-lag effect, such as the motion extrapolation account. We propose that allocentric coding of the position of the moving object could account for the flash-lag effect.     

Confusion of space and time in the flash-lag effect

The apparent lagging of a short flash in the relation to a moving object, the flash-lag effect (FLE), has so far been measured mainly in terms of illusory spatial offset. We propose a method of measuring the perceived temporal asynchrony of the FLE separately from its perceived spatial offset. We presented a moving stimulus that changed its colour at a certain moment. The observer indicated, in two different tasks, where and when the colour change occurred in relation to a stationary reference flash. Results show that the perceived time of the colour change was not congruent with the perceived location of the colour change: the colour change is perceived simultaneously with the flash, but is shifted in position. The presentation of the reference in the form of a flash is not critical for the occurrence of the FLE, because the same effect was obtained with a constantly visible reference signal, the position of which or time when it changed its colour were varied. The observer was not able to ignore the irrelevant dimension of the reference signal: the apparent time of the colour change was influenced by the position of the reference signal, and the apparent location of the colour change was influenced by the presentation time of the reference signal. The observer's inability to separate the spatial and temporal aspects of the moving stimulus clearly imposes certain limits on theories that are attempting to explain the FLE exclusively in terms of the perceived space and time.     

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.     

Retroactive contour enhancement: A new visual storage effect

A test stimulus (a visual form) which is below recognition threshold when flashed briefly against a steady background field can be raised to complete discriminability if the background field is terminated and replaced by darkness within about 100 msec of the test flash. There must therefore be an efficient storage process for the apparently “invisible” form. The phenomenon is shown to occur under a variety of conditions. It appears not to be a simple visual masking phenomenon. The tasks used are forced-choice, and the phenomenon provides a new demonstration of visual storage effects which previously have generally been reported only for supraliminal visual test stimuli.     

Experimental measures and theoretical account of hue scaling as a function of luminance

Functions relating the magnitude of chromatic response to stimulus luminance were determined at the unique green and yellow wavelengths. These measured functions were used to account for the shift from a preponderance of green at low stimulus luminances to a preponderance of yellow at higher luminances, for a stimulus wavelength of 550 nm. The two hue magnitude functions were found to have the same power-law exponents, and the hue shift was due to the difference in thresholds between the two opponent-color systems.     

Motion thresholds in infants to sinusoidal gratings.

Motion thresholds were determined at 9 degrees eccentricity in infants (mean = 14 weeks old). The stimuli used were computer-generated sinusoidal gratings presented through a 7.45 degrees aperture at a contrast ratio of .83. The range of velocities (.5, 1, 2, 4, and 6 degrees per s) was examined at only one spatial frequency (1 cycle per degree). At low velocities (less than 2 degrees per s), the infants showed no clear preference for the moving stimulus over the stationary stimulus. At faster velocities (2-6 degrees per s), the infants exhibited a clear preference for the moving stimulus. The results were interpreted as indicating that infants at 3 months of age are relatively insensitive to slow motions for low spatial frequency stimuli.     

Vibrotactile masking: Effects of oneand two-site stimulation

Masked vibrotactile thresholds at the index fingertip were measured as a function of masker intensities, which were applied to the thenar eminence of the same hand. Test and masker frequencies were selected so that the Pacinian and non-Pacinian receptor systems were selectively activated. Remote-site masking was effective only when both masker and test stimulus were within the frequency range of the Pacinian system. Cross-channel masking did not occur.