Limulus psychophysics: Increment threshold

Prior temporal summation work had indicated that the sensory code for certain behaviors (in bothLimulus and humans) can be understood if one suggests that the central nervous system analyzes the integral of the photoreceptor potential. An independent test of this suggestion is available because (inLimulus) the physiological increment threshold function obtained from the receptor potential integral is inflected, whereas that obtained from the initial transient peak of the receptor potential is not. The behavioral increment threshold function was measured inLimulus and found to be inflected. Fechner’s scaling assumption (that equally detectable stimulus increments are mediated by sensory signals of equal size) was supported by the fact that a theoretical function, which was calculated from the receptor integral intensity-response function by using Fechner’s scaling assumption, was able to fit the behavioral increment threshold function quite well. Furthermore, the variability of the behavioral data was proportional to the receptor integral variability. A seasonal effect was observed: Fall animals were more sensitive and had a higher criterion than winter animals. These findings permit the specification of the rerceptor potential that mediates this particular behavior at threshold: It has a steady-state amplitude of 7 mV in winter and 16 mV in fall. Taken in conjunction with the results of earlier temporal summation work indicating that the threshold receptor potential is 4.5 sec long, this specification implies that at least some behaviors are mediated by large, long sensory signals, which have properties very different from those of small, short signals, particularly with regard to both linearity and their relative dependence on time vs. energy.     

Eye movements during visual mental imagery

It has long been debated whether eye movements play a functional role in visual mental imagery. A recent paper by Laeng and Teodorescu presents new evidence that eye movements are stored as a spatial index that is used to arrange the component parts correctly when mental images are generated.     

Non-random sequences in visual threshold experiments

Responses (i.e. “seen” or “not seen”) to a repeated stimulus of constant intensity are shown to be grouped in runs of the same response. It is suggested that there are only two possible explanations of this non-randomness: spontaneous fluctuations of threshold and the direct influence of a response on succeeding responses. Experiments designed to distinguish between these two explanations have shown that the latter is the more important and that a response affects directly, though with diminishing intensity, the three immediately following responses.     

Eye movements and the integration of visual memory and visual perception

Because visual perception has temporal extent, temporally discontinuous input must be linked in memory. Recent research has suggested that this may be accomplished by integrating the active contents of visual short-term memory (VSTM) with subsequently perceived information. In the present experiments, we explored the relationship between VSTM consolidation and maintenance and eye movements, in order to discover how attention selects the information that is to be integrated. Specifically, we addressed whether stimuli needed to be overtly attended in order to be included in the memory representation or whether covert attention was sufficient. Results demonstrated that in static displays in which the to-be-integrated information was presented in the same spatial location, VSTM consolidation proceeded independently of the eyes, since subjects made few eye movements. In dynamic displays, however, in which the to-be-integrated information was presented in different spatial locations, eye movements were directly related to task performance. We conclude that these differences are related to different encoding strategies. In the static display case, VSTM was maintained in the same spatial location as that in which it was generated. This could apparently be accomplished with covert deployments of attention. In the dynamic case, however, VSTM was generated in a location that did not overlap with one of the to-be-integrated percepts. In order to "move" the memory trace, overt shifts of attention were required.     

Eye movements in search in visual neglect

We report data on eye movements in a patient (MP) with unilateral visual neglect, in a task of searching a real-world scene. We demonstrate that MP missed some contralesional targets after fixating on them. On such “miss”; trials, MP's eye movements were similar to those on objects in target-absent trials trials, where MP was cued to search for a target that was not actually present. In addition, MP showed little evidence of memory for the locations of objects that remained across a trial, though poor memory fails to explain performance on trials where targets were fixated. We suggest that, in this case, neglect reflected the poor uptake of information from the contralesional side of space, so that even fixated targets sometimes failed to match a “template” held in his memory.     

Rapid visual feedback processing in single-aiming movements

A major line of behavioral support for motor-program theory derives from evidence indicating that feedback does not influence the execution and control of limited duration movements. Since feedback cannot be utilized, the motor-program is assumed to act as the controlling agent. in a classic study, Keele and Posner observed that visual feedback had no effect on the accuracy of 190-msec single-aiming movements. Therefore visual feedback processing time is greater than 190 msec, and, more importantly, limited duration movements are governed by motor programs. In the present paper, we observed that visual feedback can affect the spatial accuracy of movement with durations much less than 190 msec. We hypothesize that visual feedback can aid motor control via processes not associated with intermittent error corrections.     

Evidence against visual integration across saccadic eye movements

One of the classic problems in perception concerns how we perceive a stable, continuous visual world even though we view it via a temporally discontinuous series of eye movements. Previous investigators have suggested that our perception of a stable visual environment is due to anintegrative visual buffer, a special memory store capable of fusing the visual contents of successive fixations according to their environmental coordinates. In this paper, three experiments are reported that attempted to demonstrate the existence of an integrative visual buffer. The experimental procedure required subjects to mentally fuse two halves of a dot matrix presented in the same spatial region of a display, but separated by an eye movement so that each half was viewed only during one fixation. Thus, subjects had to integrate packets of visual information that had the same environmental coordinates, but different retinal coordinates. No evidence was found in any experiment for the fusion of visual information from successive fixations in memory, leaving the status of the integrative visual buffer in serious doubt.     

Eye movements and visual imagery in free recall

The relationship between eye movements and visual imagery has almost exclusively been studied by treating eye movements as the dependent variable while an imagery task is being performed. In the present experiment three eye-movement treatment conditions were manipulated, within Ss, as the independent variable in order to study their effects on the free recall of nouns which Ss had to store by means of imagery. The imagery-evoking capacity (I) of nouns was varied over three levels within lists of Dutch nouns (Low, Medium and High I). Ss were instructed to generate a visual image to each separate noun under the following treatment conditions: (a) while they looked over and scanned their image as if they were looking at the real object: (b) while they received concurrent visual stimulation from a checkerboard pattern; (c) while they fixated on a target. Reliable, but minor effects of treatment conditions on the recall scores were found. The results were discussed in terms of possible theories about the nature of the relationship between eye movements and visual imagery.     

Evidence for visual persistence during saccadic eye movements

Summary 1. The persistence of visual perception was investigated under conditions of visual fixation as well as eye movement. The Ss' task was to discriminate brief double light impulses; their responses were recorded as a function of the duration of the interstimulus interval. Based on these data the critical interstimulus interval was calculated, which yielded equal response frequencies for the perception of one or two stimuli upon presentation of double light pulses.2. In the condition of visual fixation the two stimuli could not be discriminated until the mean value of interstimulus interval exceeded 73 msec. In the condition with eye movements, when the first stimulus was presented in the parafoveal region of the retina before the beginning of the saccade and the second stimulus in the foveal region just after termination of the eye movement, this duration was shown to be statistically of the same magnitude (76 msec).3. Possible alternative interpretations of this latter result, e.g., that it could be explained in terms of masking or saccadic suppression rather than visual persistence was discussed; it was attempted to invalidate such explanations by means of three control experiments.4. The main result, the persistence of visual perception during voluntary eye movements, was discussed in relation to the problem of spatial and temporal stability of visual perception.I thank Prof. Dr. H.W. Wendt for support in correcting the English translation.     

Eye movements can cause item-specific visual recognition advantages

Prior research suggests that spontaneous saccades localized towards blank regions of space during memory storage and recall improve memory for items at the saccade locations. In the present study, we examined whether a recognition advantage can be observed when a single, exogenously directed saccade occurs during memory maintenance. We manipulated whether participants made a saccade to an item’s previous location or maintained fixation, as well as whether tested items reappeared in their original location or not. The results of three experiments showed that visual recognition was better after a saccade to the location of a probed object than after no saccade or after a saccade to the location of a non-probed object, so long as saccades went to the to-be-tested location more often than chance. Taken together, our findings demonstrate that eye movements can elicit an item-specific recognition advantage in visual working memory.     

Saccadic eye movements and localization of visual stimuli

Visual localization phenomena were studied before, during, and after a saccade. Light flashes of 5 and 9 msec duration presented before and during the eye movement were mislocated in the saccade direction, the localization error being a time function. When the 9-msec duration stimulus and saccade did not overlap in time, a stripe was reported, when they did not, the stimulus was perceived as a point. If a long-duration stimulus moved perpendicularly to the saccade direction with the same “sigmoidal” velocity, a curvilinear trace was perceived, regardless of the linear trace of the image on the retina. A stimulus with stabilized retinal image was perceived as a stationary point during the saccade. A possible theory to deal with the data was suggested by modifying the algebra of outflow-inflow theories.     

Unimanual and bimanual continuous movements benefit from visual instructions in persons with down syndrome

ABSTRACT The authors' aim was to understand how persons with Down syndrome (DS) perform different tasks and to assess if there were any differences in performance based on the type of instructions. This is important because of neurological differences in persons with DS and neurological demands for performing different types of tasks. Twenty right-handed participants with DS, 20 chronological age-matched (CA), and 20 mental age-matched (MA) performed unimanual, bimanual, discrete, and continuous drumming following visual, auditory, and verbal instructions. Overall, discrete drumming was performed with shorter movement times than continuous drumming and unimanual drumming was performed with shorter movement amplitude than bimanual drumming. With respect to instructions, persons with DS performed with smaller amplitudes, thus more efficient movements, following the visual instructions than auditory and verbal instructions for all types of tasks, whereas CA performed similarly with all instructions and MA performed with smaller amplitudes with visual instructions than auditory instructions. These results suggest that visual instruction provides the best information for people with DS to aid in performance of many different types of movements.