Spatial interactions in the rhesus monkey retina: a behavioural study using the Westheimer paradigm

Summary

For two trained rhesus monkeys, increment thresholds for a small test-spot of 100 ms duration were determined as a function of background size, at 10 retinal eccentricities along the horizontal meridian. Typical ‘Westheimer-functions’ were obtained, i.e. threshold first increases with increasing background size, reaches a maximum, then decreases with further increasing backgrounds and finally reaches a plateau. With increasing retinal eccentricities, the position of the peak of the functions is shifted towards larger background sizes, indicating an increase of perceptive field centre size from 0.25° at 5° eccentricity to 1.5° at 40° eccentricity. The perceptive field centres tend to be slightly smaller in the nasal retina. Total perceptive field sizes, as indicated by the beginnings of the plateaus, increase from about 1° near the fovea to about 3° at 40° eccentricity. The perceptive field centre sizes of two human observers, tested under the same experimental conditions, closely resemble those of the monkeys. The total perceptive fields are larger in the human subjects. The retinal ganglion cells determining threshold in this experiment are most likely the broad-band cells. The agreement between the behaviourally determined perceptive field centre sizes and the receptive field centre sizes of broadband cells (measured by DeMonasterio and Gouras 1975) is excellent. The dendritic fields of P-α-ganglion cells, most likely the morphological substrates of the broad-band cells (Perry, Oehler and Cowey 1984) are somewhat smaller at all eccentricities. The close agreement between monkey and human psychophysical data and between monkey psychophysical and neurophysiological data indicates that the Westheimer paradigm actually provides a tool to study behaviourally the receptive field organization of the human retina.