Toshihiro Kitama

Temporal Firing Patterns of Purkinje Cells in the Cerebellar Ventral Paraflocculus During Ocular Following Responses in Monkeys I. Simple Spikes

The simple-spike firing frequency of 30 Purkinje cells (P cells) in the ventral paraflocculus (VPFL) of alert monkeys was studied in relation to vertical slow eye movements, termed ocular following response (OFR), induced by large-field visual motions of different velocities and durations. To quantitatively analyze the relationship between eye movement and firing frequency, an inverse dynamics representation of the eye movement was used for reconstructing the temporal waveform of firing. Coefficients of eye-acceleration, velocity, and position, bias, and time lag between firing and eye movement were estimated by least-square error method. In the regression analyses for each stimulus condition, 86% (146/170) of the well-modulated temporal firing patterns taken from those 30 P cells were reconstructed successfully from eye movement. The model with acceleration, velocity, and position terms, which we used, was shown as the best among several potential models by Cp statistics, consistent with t-test of significance of each term. Reliable coefficients were obtained from 75% (109/146) of the well-reconstructed firing patterns of 28 cells among 30. The estimated coefficients were larger (statistically significant) for slow stimuli than for fast stimuli, suggesting changes in sensitivities under different conditions. However, firing patterns of each cell under several different conditions were frequently well reconstructed by an inverse dynamics representation with a single set of coefficients (13 cells among 21). This indicates that the relationships between P cell firing and OFR are roughly linear in those stimulus ranges. The estimated coefficients for acceleration and velocity suggested that the VPFL P cells properly encode the dynamic components of the motor command during vertical OFR. As for the positional component, however, these P cells are correlated with eye movement in the opposite direction. In the regression analysis without positional component, remarkable differences between observed and reconstructed firing patterns were noted especially in the initial phase of the movements, indicating that the negative positional component was not negligible during OFR. Thus we conclude that, during OFR, the VPFL P cells cannot provide the necessary final motor command, and other brain regions, downstream neural structures, or other types of P cells must provide lacking position-dependent motor commands. This finding about the negative correlation with the position is in the opposite sign with previous studies obtained from the fixation and the smooth pursuit movement. From these comparisons, how the VPFL contributes to a part of the final motor command or how other brain regions complement the VPFL is suggested to be different for early and late phases of the movements.

Tonal response patterns of primary auditory cortex neurons in alert cats

The firing rates of primary auditory cortex (A1) neurons are known to be modulated only at the onset, offset, and change of a tonal stimulus in anesthetized animals. The tonal response pattern has been rarely investigated in alert animals. We investigated the time-course of A1 neuron responses to a steady tonal stimulus in alert cats. We found four types of firing responses based on statistical evaluation of the time course of the firing rate. The tonic cells (38 cells) showed a significant (P<0.05) firing increase throughout the stimulus period after a relatively long latency (mean, 25.3 ms) with little tendency of adaptation. The phasic-tonic cells (22 cells) showed a significant firing increase throughout the stimulus period after a medium latency (19.8 ms) with tendency of adaptation to less than a half of the maximum excitation level. Phasic cells (15 cells) responded, after a short latency (10.2 ms), at onset and offset of the stimuli. The unresponsive cells (26 cells) did not show a significant firing increase during stimuli. The findings suggest that there is a functional difference between each type of cells: the tonic cells encode information of static auditory signals in their firing rates; the phasic-tonic cells, of the changing auditory signal during the stimulus period; and the phasic cells, of rapid change of the auditory signal at onset and offset.

Dependence of short-latency ocular following and associated activity in the medial superior temporal area (MST) on ocular vergence.

Abstract Motion of a large-field pattern elicits short-latency ocular following responses (OFR) in the monkey, which are mediated at least in part by the medial superior temporal area of the cortex (MST). The magnitude of the OFR is known to be inversely related to viewing distance, and we investigated the dependence of OFR and the associated neuronal activity in the MST on a major cue to viewing distance, ocular vergence, in alert monkeys (Macaca fuscata). The vergence angle, expressed in terms of the apparent viewing distance, ranged from infinity to 16.6 cm (0–6 m−1). The magnitude of the initial OFR increased monotonically with increases in convergence at a mean (±SD) rate of 19.6±4.5%/m−1 in four monkeys (over the range 0–4 m−1). In two monkeys, we recorded the single unit activity of 160 MST neurons that responded to motion of a large-field pattern with directional selectivity. The mean latency (±SD) of the MST discharges elicited by large-field motion was 50±7.5 ms (n=115), which preceded the onset of OFR by an average of 10±9.9 ms. The discharge modulation elicited by large-field motion showed a significant dependence on vergence in 91/160 neurons (57%), 72 of which (79%) increased their firing rate with increasing convergence (“near” neurons), and the remainder increasing their firing rate with decreasing convergence (“far” neurons). However, on average, the sensivity of these MST neurons to vergence was only about 30% of that shown by the OFR. It could be that only those neurons that are very sensitive to vergence angle contribute to the OFR, but it is also possible that much of the modulation of OFR with vergence occurs downstream from the MST or in alternative pathways (yet to be discovered) that contribute to OFR.

Stimulation of the caudate nucleus induces contraversive saccadic eye movements as well as head turning in the cat

The effects of stimulation of the caudate nucleus were investigated in alert cats, with special reference to the induction of eye and head movements. Stimulation of caudal portions of the caudate nucleus on one side with trains of current pulses induced gaze shifts towards the contralateral side. When the head of the animal was restrained, the majority of evoked eye movements were single conjugate saccades. The amplitude and direction of the evoked saccade varied depending on the initial eye position. The amplitude of the horizontal component tended to be larger for saccades initiated from more ipsilateral positions, and became gradually smaller as the initial eye position shifted to the contralateral side. If the eye was far into the contralateral positions, no saccades were induced. Furthermore, the saccades tended to have a downward component when the eye was initially focused upward, and an upward component when the eye was focused downward. When the head was made free to move, the same stimulation induced a sequence of contraversive staircase gaze shifts composed of coordinated eye and head movements. The eye movements in the orbit resembled nystagmus, consisting of contraversive saccades followed by reverse compensatory movements. The head turning, though smooth and continuous, was also suggested to consist of a series of movements coupled with saccadic eye movements. This study indicates a potential role of the caudate nucleus in the control of orienting reflexes.

Critical spectral regions for vowel identification

The first two formant frequencies (F1 and F2) are the cues important for vowel identification. In the categorization of the naturally spoken vowels, however, there are overlaps among the vowels in the F1 and F2 plane. The fundamental frequency (F0), the third formant frequency (F3) and the spectral envelope have been proposed as additional cues. In the present study, to investigate the spectral regions essential for the vowel identification, untrained subjects performed the forced-choice identification task in response to Japanese isolated vowels (/a, o, u, e, i/), in which some spectral regions were deleted. Minimum spectral regions needed for correct vowel identification were the two regions including F1 and F2 (the first and fourth in the quadrisected F1-F2 regions in Bark scale). This was true even when phonetically different vowels had a similar combination of F1 and F2 frequency components. F0 and F3 cues were not necessarily needed. It is concluded that the relative importance in the spectral region is not equivalent, but weighted on the two critical spectral regions. The auditory system may identify the vowels by analyzing the information of the spectral shapes and the formant frequencies (F1 and F2) in these critical spectral regions.

Voiceless affricate/fricative distinction by frication duration and amplitude rise slope

Previous psychophysical studies have shown that the perceptual distinction between voiceless fricatives and affricates in consonant-vowel syllables depends primarily on frication duration, whereas amplitude rise slope was suggested as the cue in automatic classification experiments. The effects of both cues on the manner of articulation between /integral of/ and /t integral of/ were investigated. Subjects performed a forced-choice task (/integral of/ or /t integral of) in response to edited waveforms of Japanese fricatives /integral of i/, /integral of u/, and /integral of a/. We found that frication duration, onset slope, and the interaction between duration and onset slope influenced the perceptual distinction. That is, the percent of /integral of/ responses increased with an increase in frication duration (experiments 1-3). The percent of /integral of/ responses also increased with a decrease in slope steepness (experiment 3), and the relative importance between slope portions was not even but weighted at onset (experiments 1 and 2). There was an interaction between the two cues of frication duration and steepness. The relative importance of the slope cue was maximum at a frication duration of 150 ms (experiment 3). It is concluded that the frication duration and amplitude rise slope at frication onset are acoustic cues that discriminate between /integral of/ and /t integral of/, and that the two cues interact with each other.

1506 Cerebellar flocculus purkinje cell activities during head rotation in alert cats

Toshihiro Kitama, Tomohiro Omata, Yu Sato We recorded cerebellar flocculus Purkinje cell activities together with eye movements during head rotation in alert cats. The middle zone Purkinje cells were identified by simple-spike (SS) firing increase during randomdot pattern rotation to the recording side and by complex-spike (CS) firing increase during contraversive visual-pattern rotation. The vertical visual stimulation was not effective for evoking middle zone Purkinje cell responses. The CS firing was not modulated during horizontal head rotation in darkness, while the SS firing increased during ipsiversive head rotation. The gain of the vestibuleocular reflex (VOR) was less than unity. The SS responses were enhanced during head rotation together with the visual-pattern rotation to the recording side, while the CS firing increased during the contraversive head rotation. The vertical head rotation was not effective for evoking the middle zone Purkinje cell responses.

Role of premotor vestibular nucleus neurons in vertical gaze

The firing properties and projection patterns of secondary vestibular nucleus neurons involved in the vertical vestibulo-ocular pathways were investigated in alert cats. Recordings were made in the medial longitudinal fasciculus (MLF) from axons that were monosynaptically activated from the vestibular nerve. Many identified axons discharged in relation to vertical eye movements. The majority of these axons increased their firing rate for downward eye position (DPVs). During pitch rotation, the firing rate of DPVs was also related to upward head velocity, suggesting that they received monosynaptic input from the posterior canal. DPVs could be divided into two groups on the basis of their firing regularity. There was a tendency for regular DPVs to have a higher firing rate, a higher correlation for the rate-position relationship, and a larger phase lag and a smaller gain re head velocity than irregular DPVs. Spike-triggered average method and intraaxonal HRP techniques demonstrated that ipsilaterally projecting (i-) DPVs made inhibitory connections with up-on extraocular motoneurons, and contralaterally projecting (c-) DPVs made excitatory connections with down-on motoneurons. Virtually all i-DPVs were of regular type, while c-DPVs included both regular and irregular types. Stimulation of the caudal MLF at the level of the obex indicated that all the irregular c-DVPs and some of the regular c-DPVs had a collateral to the spinal cord, while none of the regular i-DPVs had such a collateral.