VISUAL-ACUITY, CONTRAST SENSITIVITY AND RETINAL MAGNIFICATION IN A MARSUPIAL, THE TAMMAR WALLABY (MACROPUS-EUGENII)

The visual acuity of the tammar wallaby was estimated using a behaviou ral discrimination task. The wallabies were trained to discriminate a high-contrast (86%) square-wave grating from a grey field of equal lum inance (1000-6000 ed m(-2)). Visual-evoked cortical potentials were us ed to measure the complete contrast sensitivity function. The stimulus was a sinusoidal phase reversal of a sinusoidally modulated grating o f various spatial frequencies and contrasts with a mean luminance of 4 0 ed m(-2). The behavioural acuity was estimated to be about 4.8 cycle s/deg. The contrast sensitivity peaked at about 0.15 cycles/deg and de clined towards both lower and higher spatial frequencies. The cut-off frequency of the contrast sensitivity function is slightly lower than the behaviourally measured acuity at about 2.7 cycles/deg. The retinal magnification factor was estimated anatomically from laser lesions to be about 0.16 mm/deg. Based on the known ganglion cell density and th e retinal magnification factor, an anatomical upper limit to visual ac uity of about 6 cycles/deg can be calculated. The differences in estim ates of visual acuity between the behavioural and anatomical methods o n the one side and physiology on the other side are discussed.