Merging of modalities in the optic tectum: infrared and visual integration in rattlesnakes.

The optic tectum of pit vipers (Crotalinae) contains a layer of infrared-sensitive neurons subjacent to the visual layer; these indirectly receive input from the facial pit organs. They respond transiently to the appearance or motion of warm objects within their 25 degrees to 70 degrees excitatory receptive fields (some have inhibitory regions) and presumably allow the snake to orient or strike toward prey. The infrared and visual spatiotopic tectal maps have similar but not identical axes; the infrared magnification is greater than that for vision. Bimodal neurons have receptive fields for each modality that reflect the disparity of the two maps. This finding suggests that (i) during development the infrared and visual fibers spread out independently to fill available tectal sites and (ii) bimodal neurons form local connections without regard to establishing spatial correspondence between the two modalities.

Cat color vision: the effect of stimulus size.

Adult cats were trained to discriminate blue from green and gray. Although the cats could discriminate the intensity of stimuli whose areas ranged from 33 to 0.36 square centimeters they could not discriminate color when the stimulus was 0.36 square centimeter (less than 20 degrees visual angle). This influence of stimulus size may account for both positive and negative results of previous studies.

Visual discriminations of cats with cortical and tectal lesions.

Fourteen cats were trained on three visual discrimination tasks: light vs. dark, horizontal vs. vertical stripes, and upright vs. inverted triangles. Four of the cats then underwent large, bilateral occipito-temporal cortex ablations; postoperatively, they demonstrated little or no visually guided orienting behavior and solve only the brightness task and not the preoperatively learned pattern discriminations. Six other cats underwent the same cortical ablations plus a transection of the commissure of the superior colliculus; postoperatively, they demonstrated good visually guided orienting behavior (i.e., the Sprague effect) but still could solve only the brightness task. The final four cats were controls and underwent no surgery; they demonstrated good retention of the pattern task despite an extensive idle period corresponding to the postoperative period before retesting the above ten cats. These data indicate that, while a transection of the collicular commissure after visual decortication dramatically improves visual orienting, it does not obviously improve visual discrimination abilities.

The effect of cortical lesions upon visual discriminations in binocularly deprived cats.

Four cats were raised with binocular eyelid suture and, after their eyes were opened, were trained on a series of discrimination tasks. They performed at normal rates on the brightness task but indicated some difficulty with the pattern tasks. They then received large, bilateral occipito-temporal cortex ablations. Postoperatively, this in no observable way affected their visually guided orienting behavior, but it did destroy their capacity to perform the preoperatively learned pattern tasks. Postoperative performance on the brightness task remained good. These data indicate that, in these deprived cats, there is little or no cortical development for visual orienting, but cortex is necessary for visual discrimination learning.

Trigeminal projections in snakes possessing infrared sensitivity.

The first order of projections of the trigeminal nerve were studied in four species of snakes, three of which are infrared sensitive, utilizing silver degeneration and cobalt iontophoresis techniques. The trigeminal nerve projected ipsilaterally to a main sensory nucleus, descending spinal nucleus, ventral trigeminal field and solitary nucleus with a contralateral projection to the base of the descending spinal nucleus in all four species. A major additional projection to the lateral nucleus of the trigeminal was present in all three infra-red sensitive species but absent in the other. Iontophoresis of cobalt chloride into a trigeminal branch composed entirely of infrared sensitive fibers and into a trigeminal branch containing no infrared fibers indicated that the lateral nucleus of the trigeminal nerve was the sole first target of the infrared system.

Anatomical and physiological localization of visual and infrared cell layers in tectum of pit vipers.

Visual and infrared cell layers were identified in the tectum of the pit vipers Crotalus viridis and Sistrurus melitus. Histologic reconstructions of 48 lesions utilizing the Prussian Blue technique were correlated with micrometer depth readings for 251 visual, infrared and bimodal single unit recordings. The visual cell layer extends caudally from approximately the level of the habenula to the rostral border of the posterior corpora quadrigemina. Neurons responding to visual stimulation are generally contained within zones 7b-13, i.e., the superficial 600--700 micrometer of the optic tectum (stratum fibrosum et griseum superficiale and the superficial sublayer of stratum griesum centrale). The infrared cell group is found in layer 7 (a and b; stratum griseum centrale) throughout the optic tectum. Eighty percent of the infrared neurons are found within 500--1,200 micrometer of the surface. In layer 7b the visual and infrared cell groups are mixed; bimodal neurons that respond to a combination of visual and infrared input are located predominantly in this sublamina. The lamination pattern for visual and nonvisual cell groups in the rattlesnake tectum appears to more closely resemble the colubrid tectum and mammalian superior colliculus than the tecta of other reptiles.

Temporal contrast sensitivity in amblyopia.

Temporal contrast sensitivity functions were determined for the normal (20/20) and amblyopic eyes of five strabismic and/or anisometropic amblyopes and for both eyes of two nonamblyopes. In deep amblyopia (20/100+), contrast sensitivity was reduced at all temporal frequencies in the amblyopic eye, whereas no deficit was observed for subjects whose acuity was 20/40 or better. This result indicates that reduced temporal sensitivity is a significant component of strabismic and anisometropic amblyopia.

Critical flicker frequency in monocularly deprived cats.

Critical flicker frequency (CFF) was determined for both eyes of long-term monocularly deprived (MD) cats over a wide luminance range. Although MD cats could discriminate flicker before and after lid opening, CFF of the deprived eye (30 Hz) was much lower than CFF of the non-deprived eye (40 Hz) and the CFF of the non-deprived eye was lower than a normal cat's monocular CFF (58 Hz). The CFF deficit of the deprived eye became less pronounced at low luminance levels. The observation (and magnitude) of a CFF deficit for the deprived eye is compatible with the reports of a Y-cell loss in LGNd. The CFF deficit of the non-deprived eye has no obvious explanation.

Increment thresholds in normal and binocularly deprived cats.

Increment thresholds on a white background were determined for normal and binocularly deprived (BD) cats over a wide luminance range. Threshold vs intensity curves had a slope of unity for both groups but the increment threshold (delta I/I) for normal cats was 0.09 while increment threshold for BD cats was 0.60. Absolute threshold was reliably better in normal cats. the deficit of BD cats is considered a potential result of their abnormal cortical physiology and/or abnormal lateral geniculate physiology.

Acuity, luminance, and monocular deprivation in the cat.

Visual acuity was determined for both eyes of long-term monocular-deprived cats over a wide luminance range. The influence of luminance on the rate of pattern vision recovery was also examined. Unlike strabismic humans and cats, the acuity deficit of monocularly deprived cats is not luminance dependent. This acuity is much worse at all luminance levels in the deprived eye. Likewise, initial testing of the deprived eye at low luminance levels does not facilitate recovery of pattern vision. Therefore, monocular lid fusion results in a visual deficit that differs from the effects of strabismus but resembles the effects of anisometropia in man.

Visual discriminations during eyelid closure in the cat.

We were able to train cats raised with sutured eyelids to perform simple brightness discriminations before their lids were parted. If, and only if, a small hole was present in a lid, could some of the cats also perform a grating orientation discrimination. By establishing their thresholds for the brightness discrimination before and after dark adaptation and before and after the lids were opened, we reached three main conclusions. (1) During dark adaptation (with pupils maximally dilated and retinae most sensitive, regardless of lid suture), the cats were 3-4 log units more sensitive with the lids open than with the lids closed. This indicates a 3-4 log unit attenuation for the lids which is in agreement with our photometric measurements. (2) During light adaptation, the sensitivity difference between the conditions of opened and closed lids was only 1-2 log units. We concluded that factors (such as pupil dilatation and retinal sensitivity) partially compensated for the lid attenuation, since the open eye could have a smaller pupil and less sensitive retina during light adaptation. (3) Given these potential compensatory features of the pupil and assuming consensual pupil sizes, the deprived eye of a monocularly sutured cat may suffer more photic deprivation (since the pupil behind the closed lid would be as constricted as the pupil in the open eye) than would either eye of a binocularly sutured cat (where both pupils can be relatively large).

Effects of posterior neocortical lesions on wavelength, light/dark and stripe orientation discrimination in ground squirrels.

Thirteen-lined ground squirrels (Citellus tridecemlineatus) were trained on three two-choice visual discrimination problems: light/dark, color and stripe orientation. After posterior neocortical lesions in one or two stages, they were tested on all three discriminations. The results demonstrate that animals with large posterior neocortical lesions which produced retrograde changes throughout the dorsal lateral geniculate (LGNd) were capable of light/dark and wavelength discrimination. These animals were not able to discriminate stripe orientation. It is proposed that wavelength discrimination depends on extrageniculostriate mechanisms in posterior neodecorticates of this species.

Visual acuity following binocular deprivation in the cat.

Threshold visual acuity for three cats which were reared from birth to 4--12 months of age with bilateral lid closure was measured and compared to visual acuity in three cats which had the use of a non-deprived eye. The results indicate that binocular deprivation (BD) results in significant deficits in visual acuity which are proportional to the duration of deprivation. Threshold visual acuities were 3.7 cycles/deg. following 4 months of BD, 3.25 cycles/deg. following 7 months of BD and 2.55 cycles/deg. following 12 months of BD compared to acuities of 6.0, 6.5 and 6.8 cycles/deg9 for cats using a non-deprived eye. All BD cats had recovered from the initial visuomotor deficits, seen in these cats and reported in the literature, following lid-parting. The implication of such deficits in visual acuity on visual discrimination learning in BD cats is discussed.

Luminance sensitivity recovers quickly in monocularly deprived cats.

We measured increment thresholds, using a reaction time or two-choice behavioral technique, in three cats monocularly deprived of normal vision for 14-16 months. Luminance increment thresholds could be obtained as early as nine days after lid opening and improved by 0.5-2.0 log units over the next few weeks. Also, visual reaction times decreased during the sensitivity improvement. When compared to the data for grating discrimination, these results suggest that visual recovery from monocular deprivation may proceed at different rates for different psychophysical discriminations.

Visual suppression and its effect upon color and luminance sensitivity.

Psychophysical increment thresholds were compared for periods of phenomenological dominance or suppression produced by different stimulation of the two eyes. Three experimental procedures were used; binocular rivalry, permanent suppression and flash suppression. The amount of suppression produced by each procedure was evaluated under conditions intended to accentuate color or luminance system contribution to the detection of a spectral flash. All three procedures resulted in a different pattern of color and luminance suppression. Binocular rivalry suppressed color sensitivity more than luminance and within color, blue (439 nm) sensitivity was more suppressed than red (613 nm). Permanent suppression resulted in a similar pattern of suppression but only blue color sensitivity was reliably more suppressed than luminance sensitivity. Flash suppression produced distinctly different results such that blue color sensitivity was reliably less suppressed than luminance or red color sensitivity, which were not different from each other. Taken together these results provide clues as to where and when the physiological processes mediating visual suppression may be found in the nervous system.

Peripheral field stimulation affects foveal flicker, but not color, sensitivity.

Human observers have different thresholds for flicker detection and color detection of a rapidly flickering spectral stimulus presented on a steady white background. A flickering surround, which did not overlap the stimulus or background, reduced flicker sensitivity but not color sensitivity for both monocular and binocular viewing. However, a flickering surround presented to one eye had no influence upon either color or flicker thresholds of the other eye.

Evidence for luminance and quasi-sustained color mechanisms.

Reaction time distributions were determined in response to near-threshold intensity increments predicted to isolate either the opponent-color or luminance system. The reaction time histograms show a clear distinction between when the chromatic and achromatic systems detect the stimulus. Our results are consistent with previous reports suggesting the achromatic system is more sensitive to higher temporal frequencies than the chromatic system.

Colour thresholds in dichromats and normals.

Studies indicate dichromats detect large, long duration spectral increments presented on bright white backgrounds with a blue-yellow colour opponent mechanism. Since opponent processes signal colour, we hypothesized that under these viewing conditions dichromats should perceive spectral increments as coloured at detection threshold. Psychophysical detection and colour discrimination thresholds were determined for normal and dichromatic humans. Test stimuli were 2 degrees, 200 ms increments presented upon a white, 1000 td, spatially coincident background. As expected, normal observers were able to discriminate between white and spectral flashes at intensities near detection threshold intensities. Dichromatic observers required suprathreshold ( approximately 0.30 log units) stimulus intensities to discriminate between the white and spectral flashes. The results do not support our hypothesis and alternative explanations for the elevated colour discrimination thresholds in dichromats are discussed.