J.H. Reuter

Tilt discrimination in the mouse

We tested the visual discrimination of mice in a two-choice discrimination box. After reaching criterion in a brightness discrimination the animals were trained on an orientation discrimination (vertical vs horizontal). Training was continued with the discrimination of vertical striations (S+) vs oblique ones (S-) of 10 different angles (80 to 15 degrees). The tilt discrimination for the last 5 days of training we found to be 30.5 (S.E.M. 3.2) degrees in albino and 19.9 (S.E.M. 0.7) degrees in pigmented mice. This is much higher than the 9.55 (S.E.M. 0.95) degrees found in pigmented rabbits but is comparable to the 17 degrees found in hooded rats by others.

The electroretinogram in normal and light-deprived rabbits.

The electroretinogram (ERG) of the rabbit develops mainly during the first four weeks after birth (NOEr~L, 1958). A cornea negative potential can be recorded at the 8th day after birth, whereas the b-wave appears between the 1 l th and 18th day. In the investigation to be presented here, the effect of postnatal light deprivation on the ERG development is studied. Two types of light stimuli were used: light flashes and sinusoidally modulated light. At first the results obtained with light flashes will be described.

Velocity-tuning of motion-sensitive and direction-selective cells in the rabbit striate cortex.

The receptive field characteristics of 268 striate cortical cells were classified in 10 Dutch-belted rabbits. From the 66 motion-sensitive and direction-selective units encountered, quantitative data describing the responses of 41 motion-sensitive, and 10 direction-selective cells to moving gratings were obtained. The results showed that all cells responded with increased spike discharges to increased velocities of the moving gratings up to a maximum, then they became less responsive to further increase of the velocities. The patterns of the response curves varied from cell to cell. When the 51 cells were taken as a whole, there were more cells responding to the velocities of 10-30 deg/sec than to higher or lower velocities. The total range over which these cells responded was 0.1-300 deg/sec. The preferred velocities for individual cells ranged from 1.5 to 150 deg/sec, showing a moderate concentration at the 6-40 deg/sec region. The extent of the effective velocities for individual cells formed a continuous distribution. No segregation into groups was seen in either of the two cell types studied or in all cells taken as a group. These data were compared with those reported for the rabbit retinal ganglion cells and the cells in the nucleus of the optic tract, as well as the cells in the cat visual system.

The contribution of the rewarded and the non-rewarded stimulus in visual discrimination learning in mice.

The relative contribution of the rewarded and non-rewarded stimulus in the learning of a visual discrimination task in various species is unclear. A number of different parameters have been suggested as influencing the importance of the one over the other, among them the type of reinforcement. Our results show that with both positive reinforcement (using water as reward after a correct choice, and no reinforcement after an incorrect choice) and negative reinforcement (electroshock punishment after an incorrect choice) mice use the position of the incorrect stimulus as a cue for the discrimination. Mice trained by positive reinforcement without the added negative reinforcement of blocking the door on which the incorrect stimulus is projected, do not learn a brightness discrimination task. This suggested that the added negative reinforcement of the closed door had a strong influence on the learning behaviour in our experiments.

Visual discrimination of discontinuous figures in the rabbit

Rabbits were trained to discriminate between vertical and horizontal striations. After the 90% correct level had been reached transfer to vertical and horizontal rows of dots was studied. It was found that performance depended on the distance between adjacent dots. With angular dot separations (measured from the centers) larger than 4.9 degrees, performance was below the 75% correct level. This result is discussed in relation to the size of the receptive fields of orientation-selective neurons in the binocular visual cortex.

Recovery from hypobaric hypoxia in the rabbit

In rabbits, recovery was studied from hypobaric hypoxia elicited by decompression to 190 mm Hg. In the first experiment a pattern discrimination habit was used. A marked latency appeared after hypoxia, before the rabbits started to respond, but once started no difference was found compared to control circumstances. Repeating this procedure 48 h later resulted in a spontaneous decrease in latency, which remained significantly longer than without hypoxia. In the second experiment rabbits were on a time-restricted daily drinking schedule and the water drinking behaviour was registered. Due to the preceding hypoxia the rate of drinking was much lower compared with the control animals, as was the average total fluid intake. After 48 h the impact of hypoxia was much less. In the third experiment the influences of repeated periods of hypoxia upon the heart rate were studied. Rabbits showed a marked bradycardia during hypoxia which was less severe 48 h later. It was argued that studying recovery from hypoxia is potentially valuable to gain information about the effects of cerebral hypoxia and the autonomic responses that influence the final outcome of hypoxic stress in the intact rabbit.

Three-dimensional retinal image stability during visual tilt discrimination in the rabbit

Rabbits trained to discriminate vertical vs. oblique striations are unable to discriminate angular differences of 5 degrees. In the present study the instabilities around the roll axis of the eyes were measured during visual discrimination. The results indicate that these instabilities are one of the causes of the rabbits inability to discriminate angular differences of 5 degrees.