Biomedical subjects

Paul S Khayat

Publications and source records attributed to Paul S Khayat.

4 recordsLinked to original sources

PubMed2006

Attention lights up new object representations before the old ones fade away.

We investigated how attention shifts from one object to another by recording neuronal activity in the primary visual cortex. Monkeys performed a contour-grouping task in which they had to select a target curve and ignore a distractor curve. Some trials required a shift of attention, because the target and distractor curves were switched during the course of the trial. We monitored the dynamics of this attention shift in area V1, in which neuronal responses evoked by the target curve are stronger than those evoked by the distractor. The reallocation of attention was associated with a rapid and strong enhancement of responses to the newly attended curve, followed, after approximately 60 ms, by a weaker suppression of responses to the curve from which attention was removed. We conclude that attention can be rapidly allocated to a new object before it disengages from the previously attended one.

Animals↗

PubMed2004

Correlates of transsaccadic integration in the primary visual cortex of the monkey.

We make several eye movements per second when we explore a visual scene. Each eye movement sweeps the scene's projection across the retina and changes its representation in retinotopic areas of the visual cortex, but we nevertheless perceive a stable world. Here we investigate the neuronal correlates of visual stability in the primary visual cortex. Monkeys were trained to make two saccades along a single curve and to ignore another, distracting curve. Attention enhanced neuronal responses to the entire relevant curve before the first saccade. This response enhancement was rapidly reestablished after the saccade, although the image was shifted across the primary visual cortex. We argue that this fast postsaccadic restoration of the attentional response enhancement contributes to the stability of vision across eye movements, and reduces the impact of saccades on visual cognition.

Animals↗

PubMed2004

Visual information transfer across eye movements in the monkey.

During normal viewing, the eyes move from one location to another in order to sample the visual environment. Information acquired before the eye movement facilitates post-saccadic processing. This "preview effect" indicates that some information is maintained in transsaccadic memory and combined with information acquired at the next fixation. However, the nature of transsaccadic memory remains a subject of debate. Here, we investigate preview effects in monkeys that carry out a contour-grouping (curve-tracing) task, by manipulating the consistency between pre- and post-saccadic information. The results show that consistent information causes a preview benefit, whereas inconsistent information causes a preview cost. These preview effects are relatively independent of the pre-saccadic viewing duration, and they occur even when the stimulus is exposed for only approximately 10 ms. The results further demonstrate that an entire relevant curve is stored in transsaccadic memory, instead of just the items at the saccade goal. They suggest that preview effects are caused by a mechanism that stores attended sensory information to make it available at the next fixation. The results are discussed within a theoretical framework that establishes an intimate relationship between attention, short-term memory and transsaccadic memory.

Animals↗

PubMed2003

Subtask sequencing in the primary visual cortex.

Complex visual tasks can usually be decomposed into a number of simpler subtasks. Whether such subtasks are solved serially or in parallel is subject to considerable debate. Here we investigate how subtasks are coordinated in time by recording from the primary visual cortex of macaque monkeys. The animals were trained to perform both a simple and a composite task. In the simple task, they had to mentally trace a target curve while ignoring a distractor curve. Neuronal responses in the primary visual cortex to the target curve were enhanced relative to responses to the distractor curve 130 ms after stimulus appearance. In the composite task, the monkeys searched for a colored marker and traced a curve that was attached to this marker. In an initial phase of the trials, neuronal responses reflected visual search, and the response enhancement due to curve tracing now occurred after 230 ms, 100 ms later than in the simple task. We conclude that subtasks of the composite task are carried out in a structured and sequential manner that can be monitored in the primary visual cortex.

Animals↗