The effects of environmental constraints upon licking patterns

Rats licked water under three degrees of environmental constraint. As restriction decreased, lick duration increased. Interlick interval changed independently of duration. Lickometer-measured drinking tube contact was longer than fluid contact unless access was highly restricted. The use of controlled access to measure minimal necessary taste input is discussed.     

How do monkeys view faces?—a study of eye movements

Face perception plays a crucial role in primate social communication. We have investigated the pattern of eye movements produced by rhesus monkeys (Macaca mulatta) as they viewed images of faces. Eye positions were recorded accurately using implanted eye coils, while neutral upright, inverted and scrambled images of monkey and human faces were presented on a computer screen. The monkeys exhibited a similar eye scan pattern while viewing familiar and unfamiliar monkey face images, or while viewing monkey and human face images. No differences were observed in the distribution of viewing times, number of fixations, time into the trial of first saccade to local facial features, and the temporal and spatial characteristics of viewing patterns across the facial images. However, there was a greater probability of re-fixation of the eye region of unfamiliar faces during the first few seconds of the trial suggesting that the eyes are important for the initial encoding of identity. Indeed, the highest fixation density was found in the eye region of all the face images. The viewing duration and the number of fixations per image decreased when inverted or scrambled faces were presented. The eye region in these modified images remained the primary area of fixation. However, the number of fixations directed to the eyes decreased monotonically from the upright images through the inverted versions to the scrambled face images. Nonetheless, the eyes remain the most salient facial substructure regardless of the arrangement of other features, although the extent of salience which they attain may depend both on the low level properties of the eyes and on the global arrangement of facial features. Electronic Publication     

Motor cortical activity in a memorized delay task

Summary Two rhesus monkeys were trained to move a handle on a two-dimensional (2D) working surface in directions specified by a light at the plane. They first captured with the handle a light on the center of the plane and then moved the handle in the direction indicated by a peripheral light (cue signal). The signal to move (go signal) was given by turning off the center light. The following tasks were used: (a) In the non-delay task the peripheral light was turned on at the same time as the center light went off. (b) In the memorized delay task the peripheral light stayed on for 300 ms and the center light was turned off 450–750 ms later. Finally, (c) in the non-memorized delay task the peripheral light stayed on continuously whereas the center light went off 750–1050 ms after the peripheral light came on. Recordings in the arm area of the motor cortex (N= 171 cells) showed changes in single cell activity in all tasks. In both delay tasks, the neuronal population vector calculated every 20 ms after the onset of the peripheral light pointed in the direction of the upcoming movement, which was instructed by the cue light. Moreover, the strength of the population signal showed an initial peak shortly after the cue onset in both the memorized and non-memorized delay tasks but it maintained a higher level during the memorized delay period, as compared to the non-memorized task. These results indicate that the motor cortex is involved in encoding and holding in memory directional information concerning a visually cued arm movement and that these processes can be visualized using neuronal population vector analysis.     

The gap effect and inhibition of return: interactive effects on eye movement latencies

Three experiments are reported with two types of manipulations that are known to affect the latency with which subjects can initiate saccadic eye movements. The first manipulation involves the temporal relation between the offset of a visual fixation point and the onset of a peripheral target (the gap effect). The second manipulation involves the prior allocation and removal of visual attention (inhibition of return). In two experiments, the gap effect was smaller for saccades to previously attended locations than to previously unattended locations. The results suggest an important link between the two phenomena and provide new insights into the brain mechanisms underlying visual attention and eye movements.     

Human 3-D aVOR with and without otolith stimulation

We describe in detail the frequency response of the human three-dimensional angular vestibulo-ocular response (3-D aVOR) over a frequency range of 0.05-1 Hz. Gain and phase of the human aVOR were determined for passive head rotations in the dark, with the rotation axis either aligned with or perpendicular to the direction of gravity (earth-vertical or earth-horizontal). In the latter case, the oscillations dynamically stimulated both the otolith organs and the semi-circular canals. We conducted experiments in pitch and yaw, and compared the results with previously-published roll data. Regardless of the axis of rotation and the orientation of the subject, the gain in aVOR increased with frequency to about 0.3 Hz, and was approximately constant from 0.3 to 1 Hz. The aVOR gain during pitch and yaw rotations was larger than during roll rotations. Otolith and canal cues combined differently depending upon the axis of rotation: for torsional and pitch rotations, aVOR gain was higher with otolith input; for yaw rotations the aVOR was not affected by otolith stimulation. There was a phase lead in all three dimensions for frequencies below 0.3 Hz when only the canals were stimulated. For roll and pitch rotations this phase lead vanished with dynamic otolith stimulation. In contrast, the horizontal phase showed no improvement with additional otolith input during yaw rotations. The lack of a significant otolith contribution to the yaw aVOR was observed when subjects were supine, prone or lying on their sides. Our results confirm studies with less-natural stimuli (off-vertical axis rotation) that the otoliths contribute a head-rotation signal to the aVOR. However, the magnitude of the contribution depends on the axis of rotation, with the gain in otolith-canal cross-coupling being smallest for yaw axis rotations. This could be because, in humans, typical yaw head movements will stimulate the otoliths to a much lesser extent then typical pitch and roll head movements.     

Excitability of reciprocal and recurrent inhibitory pathways after voluntary muscle relaxation in man

Summary We studied the potential contribution of postsynaptic mechanisms to the depression of reflex excitability which occurs immediately after a voluntary release from tonic muscle contraction. The excitability of the Soleus (Sol) motor pool was tested at rest and after voluntary muscle relaxation. In both cases the Sol H-reflex was conditioned by 1. a single shock to the peroneal nerve, in order to activate the Ia interneurones (INs) mediating the reciprocal inhibition via a peripheral input, or by 2. a short-lasting voluntary contraction of the Tibialis Anterior (TA) muscle, to activate the Ia INs via a central command. Changes in excitability of Renshaw cells were also tested at rest and after release, to assess the role of recurrent inhibition in the release-induced inhibition of the Sol H-reflex. It was demonstrated that: 1. the excitability of the INs mediating the reciprocal inhibition was only slightly enhanced in comparison with resting conditions; 2. the H-reflex of the antagonist muscle (TA) evoked after Sol release was not consistently facilitated with respect to rest; 3. the command to contract the TA muscle reduced the H-reflex of the Sol muscle during rest but not after Sol release; 4. recurrent inhibition did not increase its effect in the post-release period. Such features suggest that recurrent and reciprocal post-synaptic inhibitions do not play a major role in reducing the reflex excitability of a relaxing muscle; rather, the command to release prevents the reciprocal inhibitory effect which accompanies the contraction of the antagonist muscle. The findings support the concept that release-induced reflex depression is mediated mainly by presynaptic inhibition of autogenetic spindle afferences (Schieppati and Crenna 1984).Supported by Italian M.P.I.     

Cat red nucleus activity preceding movement depends on initiation conditions

Summary The activity of 98 Red Nucleus neurons was recorded in 3 cats operantly conditioned to perform a ballistic forelimb flexion movement triggered after a brief sound in a simple Reaction Time condition, or Delayed after the same sound in the presence of a tone cue. Fifty-eight task related neurons presented changes of activity in either one or both conditions. Forty-four of them were studied quantitatively and classified in 3 categories: 1) only 16% of the units presented similar changes of firing preceding the triggered or delayed movement; 2) most units (55%) presented different changes of activity in the two conditions: in the Delayed condition, the activation occurred earlier before the movement, and/or the change in magnitude was reduced or the pattern of activity was modified; 3) moreover, for 29% of the units, the change of activity observed before movement in the Reaction Time condition was severely reduced or even absent in the Delayed condition. For some of these neurons a building-up of activity was observed very early in the Reaction Time condition, during the preparatory period, well before the occurrence of the conditioned stimulus. These results show that the Red Nucleus activity preceding a movement is clearly dependent on its initiation conditions. The distinct patterns of unit firing observed in the Reaction Time condition and in the Delayed condition are tentatively related to the different preparation and initiation constraints determined by the behavioral conditions.     

Perceiving a stable world during active rotational and translational head movements

When a person moves through the world, the associated visual displacement of the environment in the opposite direction is not usually seen as external movement but rather as a changing view of a stable world. We measured the amount of visual motion that can be tolerated as compatible with the perception of moving within a stable world during active, sinusoidal, translational and rotational head movement. Head movements were monitored by means of a low-latency, mechanical head tracker and the information was used to update a helmet-mounted visual display. A variable gain was introduced between the head tracker and the display. Ten subjects adjusted this gain until the visual display appeared stable during sinusoidal yaw, pitch and roll head rotations and naso-occipital, inter-aural and dorso-ventral translations at 0.5 Hz. Each head movement was tested with movement either orthogonal to or parallel with gravity. A wide spread of gains was accepted as stable (0.8 to 1.4 for rotation and 1.1 to 1.8 for translation). The gain most likely to be perceived as stable was greater than that required by the geometry (1.2 for rotation; 1.4 for translation). For rotational motion, the mean gains were the same for all axes. For translation there was no effect of whether the movement was inter-aural (mean gain 1.6) or dorso-ventral (mean gain 1.5) and no effect of the relative orientation of the translation direction relative to gravity. However translation in the naso-occipital direction was associated with more closely veridical settings (mean gain 1.1) and narrower standard deviations than in other directions. These findings are discussed in terms of visual and non-visual contributions to the perception of an earth-stable environment during active head movement.     

Effects of ventrolateral thalamic nucleus cooling on initiation of forelimb ballistic flexion movements by conditioned cats

Summary Five cats were trained to perform a forelimb ballistic flexion on a reaction time paradigm including an upper limit of about 400 ms for reinforcement (food pellets). They were implanted with a cyrogenic probe thermically insulated, except at the tip, by a vacuum jacket (outer diameter, 1.1 mm).Four cats had the probe inserted into the ventrolateral thalamic nucleus (VL), contralateral to the moving limb. During cooling they showed increased reaction times, which remained constant throughout daily sessions performed during many weeks, independent of the foreperiod but varying from 25 to 100 ms according to the subject. The temperatures used to upset the reaction times varied from +10 ° C to –8 ° C, depending on the localisation of the probe and on the insulation of the silver tip used to prevent nervous tissue reaction, but for each subject the reaction times always increased when the temperature was lowered. The fifth cat, with a probe inserted between VL and the Centre Median, showed a decrease of reaction times on cooling to 0 ° C and an increase of the reaction times for a cooling at –10 ° C.For one of the four cats with a probe properly inserted into the VL, strain-gauges were stuck on the lever to measure the latency of the decrease of the pressure exerted by the subject when the subject initiated the forelimb flexion in response to the CS. Reaction times and latencies of pressure changes were closely correlated with the movement onset, and they were equally delayed during cooling. This result demonstrates that it is not by slowing down movement velocity that reaction times are upset during VL cooling but by delaying the movement onset.     

Equalization of asymmetries of tonus in the optomotor system of rabbits

Summary Turning a rabbit on a turn-table for a few degrees induces compensatory eye-movements and results in an asymmetry of tonus in the optomotor system. If the visual input is discontinued (darkness), this asymmetry decays and the eyes drift back to the mid-position within 12–18 sec. The equalization of such asymmetries of tonus under normal conditions and under curare is described. Tonus asymmetries induced by tilting the animals about the longitudinal axis are neither compensated under visual, nor under non-visual, conditions. Recordings were taken from oculomotor neurons, and changes of their firing frequencies were used as a measure for eye movements.A preliminary report was given at the spring meeting of the German Physiological Society 1973.Supported by the Deutsche Forschungsgemeinschaft, SFB 33.