The role of the human extrastriate visual cortex in mirror symmetry discrimination: a TMS-adaptation study

The human visual system is able to efficiently extract symmetry information from the visual environment. Prior neuroimaging evidence has revealed symmetry-preferring neuronal representations in the dorsolateral extrastriate visual cortex; the objective of the present study was to investigate the necessity of these representations in symmetry discrimination. This was accomplished by the use of state-dependent transcranial magnetic stimulation, which combines the fine resolution of adaptation paradigms with the assessment of causality. Subjects were presented with adapters and targets consisting of dot configurations that could be symmetric along either the vertical or horizontal axis (or they could be non-symmetric), and they were asked to perform a symmetry discrimination task on the targets while fixating the center of the screen. TMS was applied during the delay between the adapter and the test stimulus over one of four different sites: Left or Right V1/V2, or left or right dorsolateral extrastriate cortex (DLO). TMS over both Left and Right DLO reduced the adaptation effect in detecting vertical and horizontal symmetry, although the Left DLO effect on horizontal symmetry and the Right DLO effect on both vertical and horizontal symmetry were present only when considering subjects who showed a behavioral adaptation effect in the baseline No-TMS condition. Application of TMS over the Left or Right V1/V2 did not modulate the adaptation effect. Overall, these data suggest that both the Left and Right DLO contain neuronal representations tuned to mirror symmetry which play a causal role in symmetry discrimination.     

Instructions,strategies, and pattern uncertainty in a visual discrimination task

A card-sorting task involved discrimination of patterns from the same equivalence set, patterns from different sets of the same size, and patterns from different sets of different sizes. Instructions, experimental conditions, and the number of different patterns to be sorted into two trays were varied in an attempt to change Ss’ strategies from whole-pattern processing to single-element processing. Instructions alone were ineffectual in preventing whole-pattern processing, and physical masks over most of the criterion stimuli (patterns used as guides in sorting) were only somewhat effective. Differences among tasks that were attributable to size of equivalence set were eliminated only by arranging the stimuli so that a whole-pattern strategy required a great deal more information processing, or so that such a strategy could not lead to the required discrimination. The results were interpreted as confirming the influence of the properties of sets of stimuli, as well as the strength of the tendency of human Ss to process and categorize patterns in their entirety. nt]mis|This research was supported in part by a University Research Council grant.     

Stimulus correlates of visual pattern discrimination by monkeys: Multidimensional analyses

Fourteen monkeys were presented with a series of simultaneous discrimination problems, each problem consisting of two visual metric patterns, and Ss’ performance was analyzed with respect to several physical dimensions of the patterns. Proficiency of discriminative performance was found to be directly and almost exclusively related to the extent to which the two patterns, when translated, had unique parts.     

Stimulus correlates of visual pattern discrimination by monkeys: Multidimensional analyses

Fourteen monkeys were presented with a series of simultaneous discrimination problems, each problem consisting of two visual metric patterns, and Ss’ performance was analyzed with respect to several physical dimensions of the patterns. Proficiency of discriminative performance was found to be directly and almost exclusively related to the extent to which the two patterns, when translated, had unique parts.     

The effect of goodness onencoding time in visual pattern discrimination

Previous results have implied that pattern goodness, as defined by rotation/reflection equivalence set size, is a determinant of encoding time. A direct test of this implication was made via a discrete RT task using good and poor dot patterns in a two-stimulus discrimination. The following discrimination conditions were used: (1) good vs good; (2) poor vs poor; and (3) good vs poor. Overall RTs for Conditions 1 and 3 were equal, but each was significantly faster than Condition 2. In the first two conditions, there was no difference in RT within pairs and no consistent transitive ordering of the patterns. In Condition 3, however, the good pattern consistently produced the smaller RT. These results indicate that patterns of equal goodness are equally encodable, and the better the pattern, the faster it is encoded. In addition, there was evidence that pattern similarity affects RT.     

Intact visual perceptual discrimination in humans in the absence of perirhinal cortex

While the role of the perirhinal cortex in declarative memory has been well established, it has been unclear whether the perirhinal cortex might serve an additional nonmnemonic role in visual perception. Evidence that the perirhinal cortex might be important for visual perception comes from a recent report that monkeys with perirhinal cortical lesions are impaired on difficult (but not on simple) visual discrimination tasks. We administered these same tasks to nine amnesic patients, including three severely impaired patients with complete damage to perirhinal cortex bilaterally (E.P., G.P., and G.T.). The patients performed all tasks as well as controls. We suggest that the function of perirhinal cortex as well as antero-lateral temporal cortex may differ between humans and monkeys.     

Good continuation affects discrimination of visual pattern information in young infants

A familiarization-novelty preference procedure was used to determine whether 3- to 4-month-old and 6- to 7-month-old infants utilize the Gestalt principle of good continuation to organize visual patterns. The study involved presentation of a target item (i.e., a square or diamond) either in-line or off-line with a set of distractor items (i.e., circles). Infants in both age groups responded to a change in the target element in the off-line, but not in the in-line, condition. The results suggest that infants organize visual pattern information in accord with the principle of good continuation. Implications of this finding for models of the ontogenesis and microgenesis of object perception in infants and adults are discussed.