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 behavioural discrimination task. The wallabies were trained to discriminate a high-contrast (86%) square-wave grating from a grey field of equal luminance (1000-6000 cd m-2). Visual-evoked cortical potentials were used to measure the complete contrast sensitivity function. The stimulus was a sinusoidal phase reversal of a sinusoidally modulated grating of various spatial frequencies and contrasts with a mean luminance of 40 cd m-2). The behavioural acuity was estimated to be about 4.8 cycles/deg. The contrast sensitivity peaked at about 0.15 cycles/deg and declined 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 the retinal magnification factor, an anatomical upper limit to visual acuity of about 6 cycles/deg can be calculated. The differences in estimates of visual acuity between the behavioural and anatomical methods on the one side and physiology on the other side are discussed.
Hemmi
J M
JM
Research School of Biological Sciences, Australian National University, Canberra. Jan.hemmi@anu.edu.au
Mark
R F
RF
D000818:Animals; D015350:Contrast Sensitivity / Q000502:physiology*; D004193:Discrimination Learning / Q000502:physiology; D004192:Discrimination, Psychological / Q000502:physiology; D005074:Evoked Potentials, Visual / Q000502:physiology; D007614:Macropodidae / Q000502:physiology*; D008297:Male; D012160:Retina / Q000502:physiology*; D014792:Visual Acuity / Q000502:physiology*; D014793:Visual Cortex / Q000502:physiology