Psychophysical Channels and the Physiology of Perception in the Rat Whisker System

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URI: http://nbn-resolving.de/urn:nbn:de:bsz:21-opus-30523
http://hdl.handle.net/10900/45118
Dokumentart: Dissertation
Date: 2007
Language: English
Faculty: 4 Medizinische Fakultät
Department: Sonstige
Advisor: Schwarz, Cornelius PD Dr. rer.nat.
Day of Oral Examination: 2007-10-12
DDC Classifikation: 570 - Life sciences; biology
Keywords: Psychophysik , Neurophysiologie , Ratte , Tastwahrnehmung
Other Keywords:
psychophysics , neurophysiology , rat , perception , touch
License: Publishing license including print on demand
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Inhaltszusammenfassung:

Das Vibrissensystem der Ratte dient weithin als Modellsystem für sensorische Verarbeitung. Diese Dissertation beschreibt eine Präparation zur Durchführung psychophysischer Untersuchungen zur Bestimmung von Wahrnehmungsschwellen raumzeitlich präziser Auslenkungen einzelner Schnurrhaare. Eine Verhaltensstudie zur Wahrnehmungspsychophysik ergab, dass die Geschwindigkeitsschwelle für die Detektion von kleinen Auslenkungsamplituden (< 3°) deutlich höher liegt als die für größere Amplituden (> 3°). Dieser Befund wird als Hinweis auf zwei Subsysteme (psychophysische Kanäle) gewertet: das eine Subsystem (W1) vermittelt die Wahrnehmung bei großen Amplituden und relativ niedrigen Geschwindigkeiten, das andere Subsystem (W2) vermittelt bei kleinen Amplituden, benötigt aber hohe Geschwindigkeiten zur Aktivierung. Als physiologisches Substrat dieser zwei Subsysteme (= psychophysischen Kanäle) konnten zwei neuronale Zellklassen im Trigeminusganglion der Ratte identifiziert werden. Mittels Einzelzellableitung an anästhesierten Ratten konnten die langsam adaptierenden (SA) sowie die schnell adaptierenden (RA) Neurone den beiden Kanälen W1 und W2 zugeordnet werden. Im nächsten Schritt wurde das psychophysische Paradigma mit gleichzeitiger Einzelzellableitung im somatosensorischen Kortex der Ratte kombiniert. Es ergab sich, dass neurometrische Kurven der Kortexneurone ähnlich sensitiv sein können wie die gesamte Ratte, jedoch nicht signifikant sensitiver (“lower envelope principle”). Es ergab sich jedoch keine substanzielle Beziehung zwischen der Feuerrate einzelner sensitiver Nervenzellen und der Wahrnehmung der Ratte.

Abstract:

The rat whisker system has evolved into in an excellent model system for sensory processing from the periphery to cortical stages. However, to elucidate how sensory processing finally relates to percepts, methods to assess psychophysical performance pertaining to precise stimulus kinematics are needed. This dissertation describes a head-fixed, behaving rat preparation that allows to measure detectability of a single whisker deflection as a function of amplitude and velocity. A behavioral study employing the psychophysical detection task showed that velocity thresholds for detection of small-amplitude stimuli (< 3°) were considerably higher than for detection of large-amplitude stimuli (> 3°). This finding suggests the existence of two psychophysical channels mediating detection of whisker deflection: one channel exhibiting high amplitude and low velocity thresholds (W1), and the other channel exhibiting high velocity and low amplitude thresholds (W2). The correspondence of W1 to slowly adapting (SA) and W2 to rapidly adapting (RA) classes of primary afferents in the trigeminal ganglion was revealed in acute neurophysiological experiments. Neurometric plots of SA and RA cells were closely aligned to psychophysical performance in the corresponding W1 and W2 parameter ranges. Interestingly, neurometric data of SA cells fit the behavior best if it was based on a short window integrating action potentials during the initial phasic response, in contrast to integrating across the tonic portion of the response. To further elucidate sensory processing in the W1 channel, I performed neurophysiological recordings across all layers of barrel cortex in rats trained on the psychophysical detection paradigm. The whisker deflection kinematics were tailored to isolate the W1 channel. Neurometric curves derived from either single or multi unit activity in barrel cortex were less or equally sensitive to whisker deflections than the organism itself. This supports a "lower envelope" model of detection, in which perception of a faint stimulus is mediated by the most sensitive neurons available rather than average neuronal activity. In a next step, I checked whether any trial-by-trial covariation existed between neuronal activity and the report of a percept ("choice probability"). While the recordings displayed a wide range of choice probabilities, they clustered around the value expected by chance. Moreover, individual neurons' sensitivity and their associated choice probability did not show a substantial correlation. This is at odds with a lower envelope account, because this predicts that the most sensitive neurons should have the largest choice probabilities. Nonetheless, these observations suggest that the lower envelope model, while untenable in its stringent form, still provides a more satisfying description of the neural processes underlying detection of faint stimuli in the rat whisker system than alternative response pooling accounts.

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