David P. M. Northmore

Effects of eye position on auditory localization and neural representation of space in superior colliculus of cats.

The maps of visual and auditory space within the superior colliculus are in approximate register both with each other and with the underlying motor maps associated with orienting responses. The fact that eyes and ears can move independently poses a problem for the sensorimotor organization of these two modalities. By monitoring eye and pinna positions in alert, head-fixed cats, we showed that the accuracy of saccadic eye movements to auditory targets was little affected by eye eccentricity (range ±15 deg) at the onset of the sound. A possible neural basis for this behavioral compensation was suggested by recordings from superior colliculus neurons. The preferred sound directions of some neurons in the deep layers of this midbrain nucleus exhibited a shift with the direction of gaze, while in others the response throughout the auditory receptive field was either increased or decreased, suggesting that changes in eye position alter the gain of the auditory response.

Contrast sensitivity and acuity of the goldfish

Abstract Photopic contrast thresholds to sinusoidal gratings were measured in five goldfish by conditioning respiration in a Pavlovian paradigm. Vertical gratings were generated on an oscilloscope at a space-averaged luminance of 5 and 23 cd/m2. The CSF (contrast sensitivity as a function of grating spatial frequence) peaked at about 0.3 c/deg, falling off at lower and higher spatial frequencies. CSFs were fitted by double exponential functions that were used to calculate acuity and point-spread functions. Acuity, defined as the half-period of a just resolvable unity contrast grating, ranged in different fish from 13′ to 22′, the smaller value agreeing with the acuity predicted from the intercone separation. The central region of the derived point-spread function subtended about 1.6°, which is smaller than both the smallest ganglion cell receptive field centers, and the limit of full summation previously found for the light-adapted goldfish.

Behavior evoked by electrical stimulation of the hamster superior colliculus

SummarySyrian golden hamsters were implanted with fixed or moveable stimulating electrodes aimed at the superior colliculus (SC). Behavior was observed in response to trains of 0.1 ms pulses at 200 Hz while the animals were moving freely in an open arena or in their home cages. At threshold stimulating currents, the responses consisted almost entirely of freezing or contraversive turning, which occurred in two forms: fast turns, resembling orienting movements to sunflower seeds, and slow turns that were smooth and continuous. Other responses, including head raising and lowering, ipsiversive turning and backing movements were seen occasionally. Increasing the stimulating current usually gave a variety of responses, including circling movements, prolonged freezing, ipsilateral movements and running escape behavior. The sites in SC giving freezes at threshold tended to be located superficially (SO and above), or deep (SGP and below), while sites giving turns were in the intermediate layers. Most freeze sites occurred in the rostro-medial SC that represents the upper visual field, while turn sites occurred predominantly in caudo-lateral SC. Apart from the turns, most of the stimulated responses resembled natural defensive behavior, supporting the view that SC in rodents plays a role in organizing responses to predators, as well as in orienting behavior.

Vision and visual pigments in a fish, Scardinius erythrophthalmus (the rudd)

Abstract Two visual pigments may be extracted from the eye of the rudd, one based on vitamin A 1 with λ max at 507 nm, and the other based on vitamin A 2 with λ max at 535 nm. The proportion of the two pigments depends on daylength, VP507 1 increasing relative to VP535 2 with long daylengths and vice versa. Thresholds for different spectral stimuli were obtained behaviourally under photopic conditions, in a two choice discrimination situation, for fish adapted to 20 hr, 12 hr and 4 hr daylengths. The stimuli were superimposed on a tungsten light background, and photopic conditions were further maintained by an illuminated lid. Thresholds were also obtained for tungsten (“white”) light. The spectral sensitivity curves showed two clear maxima, at about 620 nm and 510 nm. There was a further less well defined maximum at short wavelengths. Spectral sensitivity was not affected by daylength, ever thought retinal extracts from fish kept under identical conditions showed marked changes in the proportions of the two visual pigments. This suggests that the pigments of the photopic receptors are unaffected by daylength, ever thought this has a large effect on the scotopic pigments. Two models of interaction between receptors at threshold were considered. It was concluded that an envelope model, where the threshold depends only on the most sensitive receptor, describes the results with both spectral and tungsten white light stimuli better than an additive model, in which it is assumed that the outputs of the different receptors summate at threshold.

VISUAL PIGMENTS FROM DIFFERENT PARTS OF THE RETINA IN RUDD AND TROUT

IN 1961 DARTNALL, LANDER and MUNZ showed that two visual pigments could be extracted from the retina of the rudd (Scardinius erythrophfhalmus), one of which was based on vitamin A,, and the other on vitamin AZ. They also found that with long daylengths the proportion of the Al-based pigment in the extracts increased, while the reverse occurred with short daylengths. A similar correlation between daylength and the relative proportion of Aland AZ-based pigments has since been demonstrated in several other teleosts (BRIDGES, 1965; BEATTY, 1966). The intensity of the upwelling light in water is always much less than the intensity of the downwelling light. Detailed measurements for lake water, for example, made by TYLER and PREISENDORFER (1962), show that the difference is often more than 2 log units. It is, therefore, possible that the superior half of the rudd retina, which receives much less light than the inferior half, also has relatively more of the AZ-based visual pigment (assuming that the effect of reducing the light intensity is the same as the effect of reducing the daylength). In fact, however, exactly the opposite was found: the proportion of Al-based pigment was greater in the superior half of the retina, A few brown and rainbow trout (Salmo fario and S. gairdneri) were also available, and retinal extracts from the superior and inferior halves of the retinas showed that the A,- and AZ-based pigments were similarly distributed in these species as well. The same effect has been independently demonstrated in the rainbow trout (REUTER, personal communication), but MUNZ and BEATTY (1965) failed to find any difference in the Coho salmon (Oncorhynchus kisutch).

Influencing and Interpreting Visual Input: The Role of a Visual Feedback System

Vertebrates are able to visually identify moving objects and orient toward attractive ones or escape if the objects seem threatening. When there is more than one object in the visual field, they can attend to a particular object. The optic tectum (superior colliculus in mammals) (OT/SC) has long

Visual and saccadic activity in the goldfish torus longitudinalis

Summary1.Multiunit activity was recorded in the torus longitudinalis (TL) of unanesthetized, immobilized goldfish while spontaneous eye movements were monitored with a search coil. Each saccade was accompanied by a burst of activity that started at the beginning of the saccade and peaked 120–150 ms later. Normally, the integrated burst amplitude was proportional to the saccade amplitude, but was independent of saccade direction or eye position. Saccadic bursts cannot be visual or proprioceptive in origin because they occur in the dark; are not evoked by passive eye movements; and persist after total paralysis with Flaxedil. Ablation of TL did not affect spontaneous eye movements.2.Photic responses, also recorded in TL, were predominantly sustained multiunit discharges to dimming of the visual field of the contralateral eye. Diffuse receptive fields were located somewhat below the field equator, and mapped naso-temporally upon the rostro-caudal axis of TL. The photic responses of TL were more sustained and of longer latency than activity recorded simultaneously in the tectum.3.Photic responses occurred more superficially in TL than the saccadic responses. Lesions of tectum abolished the photic response in the ipsilateral TL, but only attenuated the saccadic response. Lesioning the valvula, but not other parts of the cerebellum abolished the saccadic response of TL without affecting the photic response. After small lesions in the TL itself, saccadic responses were diminished, and sometimes depended upon the direction of eye movement, and the eye-in-orbit position.4.It was concluded that the photic responses in one TL derive from a topographic input from the ipsilateral tectum. These responses are probably generated by a population of neurons different from those responsible for the saccadic responses. The latter originate in the valvula of the cerebellum, which apparently transmits information about saccade direction and eye position.

The teleostean torus longitudinalis: Responses related to eye movements, visuotopic mapping, and functional relations with the optic tectum

Summary1.Multi- and single-unit recordings in the torus longitudinalis (TL) of the percoid fishes,Holocentrus andEugerres, showed two kinds of response. The first was a predominantly sustained discharge to dimming in the contralateral visual field. TL receptive fields were always found close to the equator of this field, with their naso-temporal position mapping onto the rostro-caudal axis of TL.2.The second type of activity was a bursting discharge synchronized with saccadic eye movements. Because these bursts persisted in the dark and after paralysis with Flaxedil, but were not evoked by passive eye movements, they appear to constitute a corollary discharge. Although the burst magnitude was correlated with saccade amplitude, there was no evidence for the encoding of saccade direction.3.The two response types appear to be generated by two classes of neurons because visual responses were generally recorded more superficially in TL than saccadic bursts, and the single units recorded gave either visual or saccadic responses.4.Electrical stimulation experiments provided evidence for reciprocal topographic interconnection between TL and its adjacent tectum.

Ocular dimensions and schematic eyes of freshwater and sea turtles

Measurements were made of the ocular dimensions from living and frozen eyes of one species of freshwater turtle, Pseudemys scripta elegans, and of three species of marine turtles, Chelonia mydas, Dermochelys cariacea, and Eretmochelys imbricata. Estimates of refractive error by retinoscopy were also obtained with eyes in air and under water. The results suggest that unaccommodated eyes of all four species are approximately emmetropic in air but strongly hyperopic in water. Schematic eyes were calculated for each species in both air and water.

Recovery of vision in fish after optic nerve crush: A behavioral and electrophysiological study

Recovery of the visual field was studied during regeneration of the retinotectal projection in bluegill sunfish (Lepomis macrochirus) using a behavioral perimetry technique. Fish were trained to orient ballistically to brief light flashes along the horizontal meridian. After crushing the right optic nerve intraorbitally, orienting to stimuli in the right visual field in different fish reappeared between 32 and 56 days postcrush. On average, recovery of the field center (30 to 150 degrees) occurred in 3 days, the most temporal 30 degrees requiring a further 7 days. Orientation accuracy, measured by the mean absolute error angle, returned to normal 10 to 15 days from the start of recovery. Perimetry was terminated at different times to 152 days postcrush, and the retinotectal projection was mapped electrophysiologically using an automated procedure to characterize multiunit receptive fields (MURFs). At early stages of visual recovery, MURFs associated with strong multiunit discharges were found in the same field regions responsive in behavioral perimetry. The 10- to 15-day recovery of normal orienting accuracy was associated with an increase in the amplitude of multiunit activity, attaining a plateau of 75% of normal. There was also evidence of a long-term decline in MURF area. Early maps showed a roughly normal visutopic order, although abnormalities in MURF size and shape persisted to as much as 152 days postcrush.