The spectral sensitivity of the pure‐cone retina of the grey squirrel (Sciurus carolinensis leucotis)

Abstract

The electroretinograms recorded from pure-cone retinae differ markedly from those recorded from pure-rod retinae. Thus, the pure-cone electroretinogram shows a very definite negative a-wave followed by a positive b-wave and a further positive deflexion when the stimulus is removed-the off-effect. There is no c-wave (Chaffee & Sutcliffe, 1930;Bernhard, 1941). The pure-rod or roddominated retina, on the other hand, has little or no sign of an a-wave and a b-wave immediately followed by another slow wave which may be positive or negative (Parry, Tansley & Thomson, 1953). There is often no off-effect, especially if the records are taken during dark-adaptation (Granit, 1933).It so happens that the only pure-cone retinae hitherto investigated belong to cold-blooded species, the horned toad and the tortoise, while most of the pure-, or almost pure-, rod eyes have been taken from mammals.The only mammals known to have a pure-cone retina occur in the family Sciuridae which includes the tree squirrels, ground squirrels and flying squirrels. Of these the flying squirrels are nocturnal and are said to have nearly pure-rod retinae, the ground squirrels are definitely diurnal and are said to have pure-cone retinae, while the tree squirrels are generally diurnal and have a retina which, on-purely histological grounds, probably, but not certainly, contains cones only (Walls, 1942).Since the common grey squirrel, Sciurus carolinensis leucotis, was the easiest of the diurnal Sciurids to obtain, this was the species used, in spite of the slight doubt as to the histological nature of its visual cells. We were interested to examine the general form of the electroretinogram and to determine whether this changes according to the state of adaptation, as is common for the records from mixed rod and cone retinae (Granit, 1933). We were also anxious to know whether the spectral sensitivity curve is that characteristic of rods or of cones and whether it shows a Purkinje shift.

Bleaching experiments on eyes of living grey squirrels (Sciurus carolinensis leucotis)

“…Density changes (ordinate) observed at various wavelengths (abscissa) after bleaching the squirrel eye with white light (0). The crosses ( x) represent the sensitivity, measured electroretinographically by Arden & Tansley (1955). The curve is drawn freehand.…”

“…Arden & Tansley (1955) have recently measured the spectral sensitivity of the eye of the grey squirrel, using the e.r.g. as an index of its response.…”

“…Since some authorities believe that this retina is cone-free, it became of interest to examine one which is thought to be rod-free. According to Walls (1942) and Arden & Tansley (1955), that ofthe grey squirrel (Sciurus carolinensis leucotis) contains almost certainly only cones. Accordingly, four such animals have been examined, no larger number being available.…”

Color vision in tree squirrels

“…There appear to be no cells that can be identified unequivocally as rods (Polyak, 1957). Several investigators (Arden & Tansley, 1955;Weale, 1955;Dartnall, 1960;Tansley, Copenhaver, & Gunkel, 1961) have indirectly demonstrated the existence of color vision in several species of squirrels by the shape of spectral sensitivity curves. These studies employed such techniques as electroretinography, fundus reflectometery and direct absorption measurements from extracted pigment.…”

The spectral sensitivity of the pure‐cone retina of the souslik (Citellus citellus)

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In an earlier paper (Arden & Tansley, 1955) we described the spectral sensitivity curve of the apparently pure-cone retina of the grey squirrel obtained by means of the electroretinogram. This curve was unlike those hitherto reported for any whole eye, whether rod, cone or mixed, in that it was narrow and had a maximum at about 525 my.

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