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The responses of cells in the cat's visual cortex to a moving bar of light have been analysed quantitatively, using an integration of the post-stimulus time histogram, with particular reference to orientation selectivity. The method is assessed as to its reliability and usefulness; it is shown that much precise information about a cell can be derived from its orientational tuning curve. Complex cells were found on average to show more spontaneous activity, greater response amplitude, and slightly broader orientational tuning, than either simple or hypercomplex cell types. Correlations between spontaneous and evoked activity and orientational selectivity give information as to the possible mechanisms of excitation and inhibition of the cells. The question of meridional variations in perception has been especially examined: the unexpected finding that many simple cells detecting orientations close to horizontal or vertical are very narrowly tuned, which is not the case for complex or hypercomplex cells, is discussed in relation to human psychophysical variations in orientation discrimination and contrast sensitivity. [ABSTRACT FROM AUTHOR]

The sensitivity to stimulus orientation - orientation selectivity - of simple and complex neurons in cat striate cortex was studied quantitatively. Orientation selectivity was found to be related to receptive field size - neurons with larger receptive fields being less sensitive to stimulus orientation than those with smaller receptive fields. Simple cells were more orientationally selective than complex cells. The orientation selectivity of simple cells was only weakly related to their receptive field geometry (i.e., receptive field width/length ratio), but simple cells with narrow, elongated excitatory receptive fields are more orientationally selective than those with squarer, excitatory receptive fields. These results indicate that orientation selectivity cannot be accounted for solely by the geometry of the excitatory zones and suggest that inhibitory influences sharpen the tuning of simple striate neurons for stimulus orientation. [ABSTRACT FROM AUTHOR]