CONTRAST CODING BY CELLS IN THE CATS STRIATE CORTEX - MONOCULAR VS BINOCULAR DETECTION

Many psychophysical studies of various visual tasks show that performa nce is generally better for binocular than for monocular observation. To investigate the physiological basis of this binocular advantage, we have recorded, under monocular and binocular stimulation, contrast re sponse functions for single cells in the striate cortex of anesthetize d and paralyzed cats. We applied receiver operating characteristic ana lysis to our data to obtain monocular and binocular neurometric functi ons for each cell. A contrast threshold and a slope were extracted fro m each neurometric function and were compared for monocular and binocu lar stimulation. We found that contrast thresholds and slopes varied f rom cell to cell but, in general, binocular contrast thresholds were l ower, and binocular slopes were steeper, than their monocular counterp arts. The binocular advantage ratio, the ratio of monocular to binocul ar thresholds for individual cells, was, on average, slightly higher t han the typical ratios reported in human psychophysics. No single rule appeared to account for the various degrees of binocular summation se en in individual cells. We also found that the proportion of cells lik ely to contribute to contrast detection increased with stimulus contra st. Less contrast was required under binocular than under monocular st imulation to obtain the same proportion of cells that contribute to co ntrast detection. Based on these results, we suggest that behavioral c ontrast detection is carried out by a small proportion of cells that a re relatively sensitive to near-threshold contrasts. Contrast sensitiv ity functions (CSFs) for the cell population, estimated from this hypo thesis, agree well with behavioral data in both the shape of the CSF a nd the ratio of binocular to monocular sensitivities. We conclude that binocular summation in behavioral contrast detection may be attribute d to the binocular superiority in contrast sensitivity of a small prop ortion of cells which are responsible for threshold contrast detection .