The time course of pattern discrimination in the human brain.

In electrophysiological experiments on visual pattern discrimination, decision difficulty was manipulated either via the physical characteristics of the test stimuli, or by changing the instruction given to the observer. Visual stimuli were rectangular matrices each composed of 100 Gabor patches having different orientations. Matrices differed in the number of Gabor patches with vertical, or horizontal, orientation. The observers' task was either to discriminate the dominant orientation or to detect collinear elements in the matrix. Relating task difficulty to performance, in the first experimental paradigm (detection of orientation) we obtained the conventional S-like psychometric function but in the second (detection of collinearity) the psychometric function showed a complicated U-curve. Matching between electrophysiological and psychophysical data and image statistical functions allowed us to establish the relative timing of the cortical processes underlying perception and decision making in relation to textural features. In the first 170ms after stimulus onset coding of the low-level properties of the image takes place. In the time interval 170-400ms, ERP amplitude correlated only with complex image properties, but not with task difficulty. The first effects arising from decision difficulty were observable at 400ms after stimulus onset, and therefore this is probably the earliest electrophysiological signature of the decision making processes, in the given experimental paradigm.

Head-dipping in rats with superior collicular, medial frontal cortical and hippocampal lesions.

When tested in a modified "holeboard" apparatus, hooded rats with deep superior collicular lesions were hyperactive, reared less than controls and spent a great deal of time at the apparatus perimeter. They failed to explore holes in the floor and frequently fell into them. Those with lesions restricted to superficial laminae showed reduced rearing, and preference for the perimeter but were normally active and explored floor holes as frequently as controls. The frontal cortical group failed to differ from controls on any measure. Hippocampals exhibited reduced intersession habituation of locomotor activity. It was concluded that superior colliculus, hippocampus and medial frontal cortex have separate functional roles. Differences between deep and superficial collicular lesion effects might arise because deep lesions disrupt both motor integration and attention while superficial lesions disrupt only the latter. Alternatively, since superficial lesions spare lateral tissue subserving attention to stimuli in lower portions of the visual field, they may produce a less global attentional deficiency which has less impact upon motor function.

Visual orientation in the rat: a dissociation of deficits following cortical and collicular lesions.

Rats with either bilateral ablations of superior colliculus, bilateral ablations of visual cortex, or sham operations were trained to run across a large arena towards a small illuminated target which varied in location from trial to trial. An impairment in this visually-guided running was apparent in the cortical group, but not in the collicular group. When, in a second experiment, the spatial relationships within the apparatus were changed by extending the entry-tunnel some distance into the arena, the running of the cortical group became even more impaired, while the collicular animals continued to run towards the targets under efficient visual control. In a third experiment, the effect of introducing a novel flashing light in various locations around the perimeter of the arena was investigated. It was found that unlike the other two groups, the collicular animals showed no orienting reflex to the novel stimulus when it was presented outside a broad central area of the visual field.