Minami Ito

Improvement in Visual Sensitivity by Changes in Local Context: Parallel Studies in Human Observers and in V1 of Alert Monkeys

To explore the role of primary visual cortex in contour integration, we measured the contextual sensitivity of human contrast thresholds and of superficial layer complex cells in monkey V1. An observers contrast detection was 40% improved by a second suprathreshold bar; the effect was decreased as the two bars were separated along their axis of orientation, were displaced from colinearity, and had their relative orientation changed. Recordings from V1 showed that 42% of complex cells demonstrated facilitation for a second bar outside their classical receptive fields with a similar dependency on relative location and orientation. Both effects were eliminated by an orthogonal line between the two iso-oriented lines. Multiple randomly placed and oriented lines in the receptive field surround often caused a reduction in a cells response to an optimally oriented stimulus, but this inhibition could be eliminated by changing the orientation of a few of these elements to colinearity with the centrally located target.

Attention Modulates Contextual Influences in the Primary Visual Cortex of Alert Monkeys

The response properties of cells in the primary visual cortex (V1) were measured while the animals directed their attention either to the position of the neurons receptive field (RF), to a position away from the RF (focal attention), or to four locations in the visual field (distributed attention). Over the population, varying attentional state had no significant effect on the response to an isolated stimulus within the RF but had a large influence on the facilitatory effects of contextual lines. We propose that the attentional modulation of contextual effects represents a gating of long range horizontal connections within area V1 by feedback connections to V1 and that this gating provides a mechanism for shaping responses under attention to stimulus configuration.

Interactions between attention, context and learning in primary visual cortex

Attention in early visual processing engages the higher order, context dependent properties of neurons. Even at the earliest stages of visual cortical processing neurons play a role in intermediate level vision - contour integration and surface segmentation. The contextual influences mediating this process may be derived from long range connections within primary visual cortex (V1). These influences are subject to perceptual learning, and are strongly modulated by visuospatial attention, which is itself a learning dependent process. The attentional influences may involve interactions between feedback and horizontal connections in V1. V1 is therefore a dynamic and active processor, subject to top-down influences.

Attention and Perceptual Learning Modulate Contextual Influences on Visual Perception

Brightness discrimination thresholds and facilitation by lateral interaction were measured in five human observers and two monkeys. The subjects judged the brightness of one of four peripherally seen lines against a reference. This experiment was performed both when the observer was cued to the position of the test line (focused attention) and when there was no cue (distributed attention). Discrimination was better with focused than with distributed attention. When the test line had a collinear flank, its brightness was enhanced; this enhancement was four times more prominent with distributed than with focused attention. After training, thresholds improved and collinear facilitation decreased under distributed but not under focused attention. The findings show that there are fewer benefits from contextual interaction once attention is directed toward a visual location, and that the attentional effects are subject to training.

Influence of visual saliency in monkey visual cortex

Single neuron recordings have shown that there are many color-sensitive neurons in the inferior temporal (IT) cortex, but it is still unclear how they distribute in this area. We examined color-related responses in broad brain area, including the IT cortex, by using fMRI in the alert macaque monkey. We measured the BOLD responses to chromatic stimuli (colored gratings and Mondrian-like patterns) and achromatic stimluli (same patterns without color) while the monkey was performing a fixation task. Color-biased responses were observed in LGN, V1, V2, V3 and V4. In addition, some color-biased patches were distributed in the IT cortex; around the posterior middle temporal sulcus, in the posterior bank of the superior temporal sulcus and in the anterior middle temporal sulcus. Their positions were partly overlapped with shape-biased patches detected in a separate experiment. Our results support the hypothesis that color-sensitive neurons are concentrated in multiple sub-regions in the IT cortex.

Attention modulates the contextual influences on spatial integration in V1 of awake behaving monkeys

Takafumi Akasakil, Yumiko Yoshimura” and Hiromichi Sat01 ‘Dept. of Exercise Physiology, Sch. of Health & Sport Sci., Osaka Univ., Toyonaka, Osaka 560-0043, 2Dept. of Vis. Neurosci., Res. Inst. of Environmental Medicine, Nagoya Univ., Chikusa-ku, Nagoya 464-8601. It is well-known that neurons in the primary visual cortex have stimulus-specific response properties, such as orientation tuning and length tuning. To study an integrative mechanism of information in the visual field, we examined modulatory effect on visual responses by stimulus presented outside of the receptive field of neurons in the cat visual cortex. Animals were anesthetized with a mixture of halothane (0.5 1.2 %) and N20 02 (2:1), and paralyzed with pancuronium bromide. Responses to square-wave or sinusoidal grating stimulus presented on the receptive field were modulated by grating stimulus outside of the receptive field. The response modulation was basically suppressive and strength of the modulation was dependent on orientation contrast of stimuli inside and outside of the receptive field. The effects were more often observed in layer II/III and rarely in layer IV, and they seemed to be partly, at least, due to orientation -dependent spatial interaction within and near the classical receptive field.