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PubMed1987

Perception of synthetic visual consonant-vowel articulations.

Synthetic speech-like articulations were presented to adult subjects via the visual modality, following the classic categorical perception experimental paradigm (Liberman, Harris, Hoffman, & Griffith, 1957). Animations were generated on a computer-based graphics system. Stimuli consisted of representations of the syllables /ba/, /va/, and /wa/; as well as 6 linearly interpolated intermediate stimuli between each of the possible exemplar pairs, resulting in three 8-item continua. Three sets of observations were obtained for these stimuli. First, for each continuum, labeling data were obtained in which the subject assigned one or the other exemplar label to each of the stimuli. Next, ABX discrimination data were obtained for each continuum. In the final task, subjects assigned a rating of one through nine to each animation indicating the extent to which it was like the exemplar syllables. Although the labeling functions showed rather abrupt transitions from one response category to the other, the peaks in the discrimination functions did not coincide with the category boundaries. Further, the mean rating functions were relatively linear, and the distribution of rating responses revealed unimodal distributions whose peak locations differed depending on the stimulus.

Adult↗
PubMed2003

Augmentation of induced visual gamma activity by increased task complexity.

Recently the study of induced gamma band oscillations has focused on their modulation by top-down processes, mainly attention. Numerous studies have observed an increase in induced gamma band energy with increases in covert selective attention and visual perception. The current study investigated the modulation of visually induced gamma band oscillations by top-down processes associated with task complexity. Fourteen human subjects performed a reaction time task under two experimental conditions that differed in task complexity. In one, subjects simply had to press one of four buttons that corresponded to a colour stimulus shown to the subject. In the second, the stimulus response mapping was altered by the implementation of a rule, thus increasing task complexity. Cortical electrical activity was recorded using a 65 electrode whole scalp electroencephalographic (EEG) net. The EEG activity was analysed using Morlet wavelets to produce time-frequency maps. Although induced gamma band activity was observed in both conditions, there was significantly greater energy during the rule-operation condition at approximately 276 ms after the appearance of the stimulus. This increase was localized to electrodes overlying the right-central parietal scalp. The results of this study show that top-down processes modulate the level of induced gamma band activity. We discuss these findings in terms of the role of gamma oscillations in the construction of a sensory representation useful for a correct motor response.

Adolescent↗
PubMed1996

Remote operation: a selective review of research into visual depth perception.

Some perceptual motor operations are performed remotely; examples include the handling of life-threatening materials and surgical procedures. A camera conveys the site of operation to a TV monitor, so depth perception relies mainly on pictorial information, perhaps with enhancement of the occlusion cue by motion. However, motion information such as motion parallax is not likely to be important. The effectiveness of pictorial information is diminished by monocular and binocular information conveying flatness of the screen and by difficulties in scaling: Only a degree of relative depth can be conveyed. Furthermore, pictorial information can mislead. Depth perception is probably adequate in remote operation, if target objects are well separated, with well-defined edges and familiar shapes. Stereoscopic viewing systems are being developed to introduce binocular information to remote operation. However, stereoscopic viewing is problematic because binocular disparity conflicts with convergence and monocular information. An alternative strategy to improve precision in remote operation may be to rely on individuals who lack binocular function: There is redundancy in depth information, and such individuals seem to compensate for the lack of binocular function.

Depth Perception↗
PubMed2004

The role of perceptual bias in complex figure recall.

Several studies have demonstrated that constructional and organizational abilities affect how well visual stimuli are remembered. The purpose of the present study was to determine whether perceptual bias, independent of constructional ability, influenced visual memory performance. Participants were 118 normal adults who were administered the Rey-Osterrieth Complex Figure (ROCF) and a similarity judgement task requiring then to decide which of two hierarchically constructed comparison figures most resemble a standard figure. The initial steps of a multiple regression analysis indicated that delayed recall of the ROCF was directly related to copy accuracy, inversely related to age, and better in males than females. In the final step of the regression model, performance on the similarity judgement task also predicted ROCF recall, with a more global perceptual bias predicting better recall. Results indicate that even when copy accuracy and demographic variables are accounted for, components of visual perception influence how well visual stimuli are encoded and recalled. Findings are consistent with research correlating global processing and spatial ability, and suggest that in addition to copy ability and organization, visuoperceptual bias must be considered as a moderator variable when interpreting performance as a visual memory test.

Adult↗
PubMed1991

Simultanagnosia. To see but not two see.

Simultanagnosia is a disorder of visual perception characterized by the inability to interpret complex visual arrays despite preserved recognition of single objects. We report a series of investigations on a simultanagnosic patient which attempt to establish the nature of this visual processing disturbance. The patient performed normally on a feature detection task but was impaired on a test of attention-requiring visual search in which she was asked to distinguish between stimuli containing different numbers of targets. She was not impaired on a visual-spatial orienting task. She identified single briefly presented words and objects as rapidly and reliably as controls suggesting that access to stored structural descriptions was not impaired. With brief, simultaneous presentation of 2 words or drawings, she identified both stimuli significantly more frequently when the stimuli were semantically related than when they were unrelated. On the basis of these and other data, we suggest that the patient's simultanagnosia is attributable to an impairment in the process by which activated structural descriptions are linked to information coding the location of the object.

Aged↗