Perception: Dr Matt Mundy: Weeks 1-6Cognition: VariousUnit coordinator: Dr Matt Mundy (Rm 532, Building 17, Clayton)Assistant coordinator: Anne-Marie Ternes (AnneMarie.Ternes@monash.edu)Tel: 03990 51035 (only for urgent enquiries)Consultation times: By appointmentE-mail: matthew.mundy@monash.edu

Moodle News and announcements important notices Unit guide Lecture and lab content Discussion forums Should be your primary way to ask general questions. Look here for the answers to commonly asked questions before contacting your Tutor/Anne Marie/Me!

Outline of perception lectures Perception: first 6 teaching weeks of semester (2 hrs/wk.) Lecture summaries available on the web at PSY3051 Moodle site. Lecture recordings will be available on Wednesday afternoons via MULO. Textbook: PSY3051/PSY4081 Perception and Cognition. Bruce Goldstein.Compiled by Matthew Mundy. Monitored discussion forum:Access via Moodle use the forum most appropriate to your question Twitter: Follow @DrMattMundy to post questions before, during and after the lecture, or use #PSY305

Outline of perception lectures Week 1: Introduction & visual processing Week 2: Visual processing (higher centres) Week 3: Perceiving objects, scenes & and colour Week 4: Perceiving depth, size & illusions Week 5: Hearing: the auditory system, localisation & auditory scene analysis Week 6: Speech perception

Introduction to perception, what is perception? A largely unconscious, automatic process, based on unavailable neural events, together with unconscious inferences from specific cues But, at times conscious effort is needed to interpret sensory data (e.g., when data are ambiguous and incomplete) Effortlessness of perception disguises the complex nervous system mechanisms operating (behind the scenes)

Two Interacting Aspects of Perception Bottom-up processing Processing based on incoming stimuli from the environment Also called data-based processing Top-down processing Processing based on the perceivers previous knowledge (cognitive factors) Also called knowledge-based processing

Approaches to the Study of Perception Observing perceptual processes at different stages in the system:>Psychophysical approach (PP) - the stimulus-perception relationship>Physiological approach (PH1) - the stimulus-physiology relationship>Physiological approach (PH2) - the physiology and perception relationship These stages are interconnected and communicate with one another.

Psychophysics - Overview of Methods of Measurement Qualitative Methods Describing Recognising Quantitative Methods Detecting Perceiving Magnitude Searching

Qualitative Methods of Psychophysical Measurement Description Indicating characteristics of a stimulus First step in studying perception Called phenomenological method Recognition Placing a stimulus in a category by identifying it Categorisation of stimuli Used to test patients with brain damage

Quantitative Methods Classical Psychophysics Absolute threshold - smallest amount of energy needed to detect a stimulus Method of limits>Stimuli of different intensities presented in ascending and descending order>Observer responds to whether she perceived the stimulus>Cross-over point is the threshold

Classical Psychophysics - continued Absolute threshold (cont.) Method of adjustment>Stimulus intensity is adjusted continuously until observer detects it>Repeated trials averaged for threshold

Classical Psychophysics - continued Absolute threshold (cont.) Method of constant stimuli> Five to nine stimuli of different intensities are presented in random order>Multiple trials are presented detection yes or no>Threshold is the intensity that results in detection in 50% of trials.

Classical Psychophysics - continued Difference Threshold or Limen (DL) -smallest difference between two stimuli a person can detect Same methods can be used as for absolute threshold As magnitude of stimulus increases, so doesDL Webers Law describes this relationshipDL / S = K>Webers Law doesnt hold over whole range of sensation magnitudes

Quantitative Methods Modern Psychophysics Magnitude estimation (scaling) Stimuli are above threshold. Observer is given a standard stimulus and a value for its intensity. Observer compares the standard stimulus to test stimuli by assigning numbers relative to the standard.

Modern Psychophysics - continued Magnitude estimation (cont.) Response compression>As intensity increases, the perceived magnitude increases more slowly than the intensity. Response expansion>As intensity increases, the perceived magnitude increases more quickly than the intensity.

Quantitative Methods - continued Magnitude estimation (cont.) Relationship between intensity and perceived magnitude is a power function Stevens Power Law>P = kSn

Basic Brain Structure The brain has modular organization The sensory modalities have primary receiving areas>Vision - occipital lobe>Audition - temporal lobe>Tactile senses - parietal lobe Frontal lobe coordinates information received from two or more senses

Neurons: Transduction & Transmissiona very brief recap! Key components of neurons: Cell body Dendrites Axon or nerve fibre Receptors - specialized neurons that respond to specific kinds of energy

Neural Signals Electrical signals or action potentials occur when: Permeability of the membrane changes Na+ flows into the fibre making the neuron more positive, then, K+ flows out of the fibre making the neuron more negative Finally Na+ pumped out of axon, to restore normal cell level. This process travels down the axon in a propagated response

Properties of Action Potentials Action potentials: Show propagated response. Remain the same size regardless of stimulus intensity. Increase in rate to increase in stimulus intensity. Have a refractory period of 1 Ms - upper firing rate is 500 to 800 impulses per second. show spontaneous activity that occurs without stimulation.

Synaptic Transmission of Neural Impulses Neurotransmitters are: released by the presynaptic neuron from vesicles. received by the postsynaptic neuron on receptor sites. matched like a key to a lock into specific receptor sites. used as triggers for voltage change in the postsynaptic neuron.

Types of Neurotransmitters Excitatory transmitters - cause depolarization Neuron becomes more positive Increases the likelihood of an action potential Inhibitory transmitters - cause hyperpolarization Neuron becomes more negative Decreases the likelihood of an action potential

Neural Circuits - in the eye Groups of neurons connected by excitatory and inhibitory synapses A simple circuit has no convergence and only excitatory inputs. Input into each receptor has no effect on the output of neighbouring circuits. Each circuit can only indicate single spot of stimulation.

Neural Circuits - continued Convergent circuit (excitatory connections) Input from each receptor summates into the next neuron in the circuit. Output from convergent system varies based on input. Output of circuit can indicate single input and increases output as length of stimulus increases.

Neural Circuits - continued Convergent circuit with excitatory and inhibitory connections Inputs from receptors summate to determine output of circuit. Summation of inputs result in:>weak response for single inputs and long stimuli.>maximum firing rate for medium length stimulus.

Receptive Fields Area of receptors that affects firing rate of a given neuron in the circuit Receptive fields are determined by monitoring single cell responses. Research example for vision Stimulus is presented to retina and response of cell is measured by an electrode.

Center-Surround Receptive Fields Excitatory and inhibitory effects are found in receptive fields. Center and surround areas of receptive fields result in: Excitatory-centre-inhibitory surround Inhibitory-centre-excitatory surround

Center-Surround Antagonism Output of centre-surround receptive fields changes depending on area stimulated: Highest response when only the excitatory area is stimulated Lowest response when only the inhibitory area is stimulated Intermediate responses when both areas areStimulated

Convergence in the Retina - continued 126 million rods and cones converge to 1million ganglion cells. Higher convergence of rods than cones Average of 120 rods to one ganglion cell Average of six cones to one ganglion cell Cones in fovea have one to one relation to ganglion cells

Convergence and Sensitivity Rods are more sensitive to light than cones. Rods take less light to respond Rods have greater convergence which results in summation of the inputs of many rods into ganglion cells increasing the likelihood of response. Trade-off is that rods cannot distinguish detail

Convergence and Detail All-cone foveal vision results in high visual acuity One-to-one wiring leads to ability to discriminate details Trade-off is that cones need more light to respond than rods

Lateral Inhibition of Neurons Experiments with eye of Limulus (Horseshoe Crab) Ommatidia allow recordings from a single receptor. Light shown into a single receptor leads to rapid firing rate of nerve fibre. Adding light into neighbouring receptors leads to reduced firing rate of initial nerve fibre

Lateral Inhibition and Lightness Perception Three lightness perception phenomena explained by lateral inhibition The Hermann Grid: Seeing spots at an intersection Mach Bands: Seeing borders more sharply Simultaneous Contrast: Seeing areas of different brightness due to adjacent areas

Hermann Grid People see an illusion of Gray images in intersections of white areas. Signals from bipolar cells cause effect Receptors responding to white corridors send inhibiting signals to receptor at the intersection The lateral inhibition causes a reduced response which leads to the perception of Gray.

Mach Bands People see an illusion of enhanced lightness and darkness at borders of light and dark areas. Actual physical intensities indicate that this is not in the stimulus itself. Receptors responding to low intensity (dark) area have smallest output. Receptors responding to high intensity (light) area have largest output.

Mach Bands - continued All receptors are receiving lateral inhibition from neighbours In low and high intensity areas amount of inhibition is equal for all receptors Receptors on the border receive differentialInhibition

Mach Bands - continued On the low intensity side, there is additional inhibition resulting in an enhanced dark band. On the high intensity side, there is less inhibition resulting in an enhanced light band. The resulting perception gives a boost for detecting contours of objects.

Sensory Code: Representation of Environment Sensory code - representation of perceived objects through neural firing Specificity coding - specific neurons responding to specific stimuli>Leads to the grandmother cell hypothesis>Recent research shows cells in the hippocampus that respond to concepts such as individual celebrities.

Sensory Code: Representation of Environment -continued Problems with specificity coding:>Too many different stimuli to assign specific neurons>Most neurons respond to a number of different stimuli. Distributed coding - pattern of firing across many neurons codes specific objects Large number of stimuli can be coded by a few neurons.