A Peri-Stimulus Time Histogram (PSTH) is a graphical tool widely used in neuroscience. It illustrates how the firing pattern of a single neuron, or brain cell, changes in response to a specific event or stimulus. By displaying the timing of electrical signals, known as spikes, a PSTH allows scientists to observe the neuron’s activity. This method helps researchers understand the functional role of individual neurons within the brain, providing a visual summary of their average response.
Constructing a PSTH
Creating a Peri-Stimulus Time Histogram involves a precise sequence of steps to transform raw neural recordings into a meaningful display. Scientists first record the electrical signals, or “spikes,” generated by a neuron across many repetitions of the same event or stimulus.
For each instance of the stimulus presentation, also known as a trial, the recorded neural activity is aligned to the exact moment the stimulus occurred. This specific point in time is designated as “time zero” on the recording. The time intervals both before and after this zero point are then divided into small, equal segments called “bins,” which commonly range from 1 to 10 milliseconds in duration.
The next step involves counting the total number of spikes that fall within each of these predefined time bins across all the collected trials. This count is then converted into an average firing rate by dividing the total spike count in each bin by the total number of trials and the width of the bin. The result is expressed in spikes per second. This calculated average firing rate for each bin is then plotted to form the histogram.
Interpreting a PSTH Plot
Understanding a PSTH plot involves examining its two primary axes and the patterns displayed. The horizontal, or x-axis, represents time relative to the stimulus onset, with time zero marking the precise moment the stimulus was presented. Negative values on this axis indicate time before the stimulus, while positive values represent time after it. The vertical, or y-axis, shows the average firing rate of the neuron, usually measured in spikes per second or as a probability of spike occurrence.
The segment of the graph before time zero illustrates the neuron’s baseline activity, which is its typical firing rate when no specific stimulus is present. Following the stimulus, distinct shapes in the histogram signify different neuronal responses. A noticeable peak appearing after time zero indicates an excitatory response, meaning the neuron increased its firing rate in reaction to the stimulus. Conversely, a dip below the baseline level suggests an inhibitory response, where the neuron’s activity decreased.
Applications in Neuroscience Research
Peri-Stimulus Time Histograms are widely applied across various fields of neuroscience to gain insights into brain function. In sensory processing, PSTHs can reveal how neurons in the visual cortex respond to specific visual stimuli. A neuron might show a sharp increase in firing rate when a light stimulus appears at a specific angle or location, helping map how the brain interprets visual information.
PSTHs are also used to understand motor control. Researchers can record from neurons in the motor cortex and observe their PSTHs as a subject prepares for a movement. A neuron’s firing rate might consistently increase before the hand movement begins, indicating its involvement in planning and executing the action, linking neuronal activity to volitional behaviors.
In research on learning and memory, PSTHs can track changes in neural responses over time as an animal learns new associations. If an animal learns that a sound predicts a reward, a neuron’s PSTH to that sound might initially show a weak response but then develop a strong activation after training, demonstrating how neuronal circuits adapt. These applications contribute to a deeper understanding of brain mechanisms underlying perception, action, and cognition.
PSTHs and Raster Plots
While Peri-Stimulus Time Histograms offer an averaged view of neuronal activity, they are often paired with another visualization tool called a raster plot to provide a more complete picture. A raster plot displays individual neuronal spikes across multiple trials of an experiment. Each horizontal line on a raster plot represents a single trial, and each tick mark indicates the precise moment a neuron fired an electrical impulse during that trial.
The PSTH summarizes the average firing pattern by combining data from all trials into a single histogram, showing when a neuron is most likely to spike. In contrast, the raster plot preserves the trial-to-trial variability and the exact timing of each spike.
Neuroscientists frequently examine a raster plot positioned directly above its corresponding PSTH. This combined view provides both the overall trend of the neuron’s response (from the PSTH) and the detailed, raw data that contributed to that average (from the raster plot).