Spatial and temporal properties of human rod vision in the achromat

The spatial and temporal properties of rod vision were measured for stimuli at and above the detection threshold in an achromat whose spectral sensitivity, dark adaptation, spatial and temporal thresholds and Stiles-Crawford effect suggest the presence of only a normally functioning rod system. The properties of rod and cone vision were compared at illuminances where their respective sensitivities were optimum. The threshold spatial sensitivity of the rod mechanism under optimum illumination (180 scotopic trolands) exhibits bandpass properties with a peak sensitivity of around 80 at 0.5 cycles/deg and a spatial acuity of 6-7 cycles/deg. The threshold temporal sensitivity also exhibits bandpass properties under these conditions with a peak sensitivity of around 80 at 5 Hz and a temporal acuity of 30 Hz. For stimuli of low spatial frequency (less than 0.3 cycles/deg) and low temporal frequency, the threshold sensitivities of rod- and cone-mediated vision are identical. Rod- and cone-mediated vision display comparable spatial and temporal discrimination for targets of equal suprathreshold contrast over the low to mid spatial and temporal range that they share. Rod-mediated discriminations fall below those of cone vision above 1 cycle/deg for spatial judgements and above 15 Hz for temporal judgements. The number of discriminable steps in spatial frequency and temporal frequency at threshold is similar for rod and cone vision over the spatio-temporal frequency range that they share. Over this range rod- and cone-mediated vision can discriminate four steps in spatial frequency and one step in temporal frequency. These results suggest that rod vision shows comparable spatio-temporal discrimination performance to cone vision and that it is subserved by at least five spatial and two temporal labelled detectors. The response of the highest spatial frequency filter subserving rod vision extends from 0.5 to 6 cycles/deg.