What do you notice? Woodman, Atten. Percept. Psychophys., 2010

Transcription

1 What do you notice? Woodman, Atten. Percept. Psychophys., 2010

2 You are trying to determine if a small amplitude signal is a consistent marker of a neural process. How might you design an experiment to test this?

3 Idealized ERP evoked from auditory stimulus: Event-related potentials: electrical potentials evoked in response to an event (e.g., a stimulus), measured using electroencephalography (EEG). ERPs have a small amplitude relative to background EEG oscillations. Woodman, Atten. Percept. Psychophys., 2010 Signals are aligned to events and averaged over many trials.

4 Luck, MIT Press, 2005

5 Extracting a signal from the noise can require thousands of trials: In this example, the noise during the baseline is the same amplitude as the observed task-related differences. In addition the initial prominent components of the ERP create a waveform with the same period as the alpha rhythm. Distinguishing ERP components from the alpha rhythm can require averaging across thousands of trials. Woodman, Atten. Percept. Psychophys., 2010

6 Consistency of the ERP across subjects: Idealized ERP during a visual task: Woodman, Atten. Percept. Psychophys., 2010 N2pc occurs specifically over visual cortex in response to attended stimuli. Luck, MIT Press, 2005

7 The EEG components: Defined by their polarity, latency (the time or order of occurrence after the onset of an event), their scalp distribution and their sensitivity to task dimensions. Idealized ERP during a visual task: P = positive components N = negative components 1,2,3 = the order in which they occur Components can also be referred to by their latency (e.g. P100). N2pc occurs specifically over visual cortex in response to attended stimuli. Important: each component label refers to a family of components, not necessarily related

8 The exogenous EEG components: Early sensory responses that depend on external factors. Idealized ERP during a visual task: Examples: C1 = the first visual component, with varying polarity depending upon whether stimuli are in the upper or lower visual field. P1 = first positive peak, influenced by stimulus parameters such as luminance.

9 The exogenous EEG components: Early sensory responses that depend on external factors. Idealized ERP evoked from auditory stimulus: I-VI = brainstem auditory evoked responses (BAERs) N 0 -Nb = thought to arise in part from activity between the medial geniculate nucleus (MGN, thalamus) and the cortex Woodman, Atten. Percept. Psychophys., 2010

10 The endogenous EEG components: Relatively late sensory responses that depend on internal factors. P3(b) = amplitude is dependent upon factors such as task difficulty and stimulus probability, and not dependent upon the physical properties of the eliciting stimulus Vogel et al., J. Exp. Psych. Hum. Percept. Perform., 1998

11 The endogenous EEG components: P300 probability of stimulus class and associated psychological operation : Experiment 1: Experiment 2: Homogeneous = count fixed target letter mixed in with As 80% As, 10% Bs, 10% either letters (C-Z) or numbers (0-23) Heterogeneous = count fixed target letter mixed in with any other letter Dotted = Bs Thick = letters Thin = numbers Response to target is greater even when non-targets are less frequent Average ERPs for 9 subjects, identical P300s in homogeneous and hetergeneous conditions Task-related probability as opposed to absolute probability Courchesne et al., Psychophys., 1977

12 You are trying to determine if a small amplitude signal is a consistent marker of a neural process. How might you design an experiment to test this?