Midbrain Periaqueductal Gray and Vocal Patterning -- Spike Train Analysis.

Matt Kittelberger, Bruce Land, Andy Bass

Abstract from paper (1)

Midbrain structures, including the periaqueductal gray (PAG), are essential nodes in vertebrate motor circuits controlling a broad range of behaviors, from locomotion to complex social behaviors such as vocalization. Few single unit recording studies, so far all in mammals, have investigated the PAG's role in the temporal patterning of these behaviors. Midshipman fish use vocalization to signal social intent in territorial and courtship interactions. Evidence has implicated a region of their midbrain, located in a similar position as the mammalian PAG, in call production. Here, extracellular single unit recordings of PAG neuronal activity were made during forebrain-evoked fictive vocalizations that mimic natural call types and reflect the rhythmic output of a known hindbrain-spinal pattern generator. The activity patterns of vocally active PAG neurons were mostly correlated with features related to fictive call initiation. However, spike trains in a subset of neurons predicted the duration of vocal output. Duration is the primary feature distinguishing call types used in different social contexts, and these cells may play a role in directly establishing this temporal dimension of vocalization. Reversible, lidocaine inactivation experiments demonstrated the necessity of the midshipman PAG for fictive vocalization, while tract-tracing studies revealed the PAG's connectivity to vocal motor centers in the fore- and hindbrain comparable to that in mammals. Together, these data support the hypotheses that the midbrain PAG of teleosts plays an essential role in vocalization and is convergent in both its functional and structural organization to the PAG of mammals.

The process

For each unit confirmed by histology to be in the PAG (see below), we calculated:

  1. mean spontaneous firing rate (from trials with no stimulus)
  2. net mean number of stimulus-evoked spikes (over the first 800 msec post-stim, reduced by the number of expected spontaneous spikes)
  3. mode of the inter-spike interval distribution
  4. latency to the first post-stimulus spike
  5. mean lag time between the first post-stimulus unit spike and the first pulse of the vocal response

In addition, for each of these fish, we determined various parameters of the vocal response, including:

  1. total mean number of pulses (over the first 800 msec post-stimulus)
  2. mean number of pulses in the first vocal burst (a burst was defined as a series of pulses, each separated from the
    next by no more than 50 msec)
  3. latency to the first pulse of the vocal response
  4. mean inter-pulse interval
  5. mean inter-burst interval.
  6. post-stimulus time histograms (PSTH) for each unit and vocal response
  7. peri-event time histograms (PETH) for each unit (centered on the time of the first vocal response pulse)
  8. frequency distributions of inter-spike intervals (for each unit and vocal response)

The programs

All programming was done in Matlab 6.5 from the Mathworks. The data was handled in two program steps. The raw text files were parsed from the raw data acqusition format by this program, and converted to tab-delimited (Excel compatable), human readable files. These files were then read by the main analysis program which handled both single unit and nerve data. Program comments are shown below:

%A 'b' in the file name means single unit
%--no b means whole nerve recording
%
%Data handling:
%--for vocal nerve, discard spikes before stim end
%--for single unit, subtract spikes before stim start
%--stim length is (Npulse-1)*3+1 mSec (col 3 on spread sheet)
%--time before stim is always 200 mSec
%
%For Vocal nerve:
% #spikes/sweep, average #/set, average #/group
% latency to onset (by sweep, set, stim group)
% ISI -> freq plot and mean ISI
% IBI and # of Bursts (threshold for burst start 50 mSec)
% #spikes/burst
%
%For single unit:
% #spikes/sweep, average #/set, average #/group
% subtract off the numbers from the zero stim sets
%separate PST section

Reference

(1) Kittelberger, J. Matthew, Land, Bruce, Bass, Andrew (in press 2006)
The Midbrain Periaqueductal Gray and Vocal Patterning in a Teleost Fish. Journal of Neurophysiology

Copyright Cornell University, 2006