Midterm 1 Oct. 21 in class Read this article by Wednesday next week! Mental Representations • Mental representations can start with sensory input and progress to more abstract forms – Local features such as colors, line orientation, brightness, motion are represented at low levels A “labeled line” -Activity on this unit “means” that a line is present -Does the line actually have to be present? Mental Representations • Mental representations can start with sensory input and progress to more abstract forms – texture defined boundaries are representations arrived at by synthesizing the local texture features Mental Representations • Mental representations can be “embellished” - Kaniza Triangle is represented in a way that is quite different from the actual stimulus -the representation is embellished and extended Mental Representations • Mental Representations can be transformed – Rubin Vase, Necker Cube are examples of mental representations that are dynamic Mental Representations • Mental Representations can be transformed – Shepard & Metzlar (1971) mental rotation is an example of transforming a mental representation in a continuous process Mentally rotate the images to determine whether they are identical or mirror-reversed SAME MIRROR-REVERSED Mental Representations • Mental Representations can be transformed – Shepard & Metzlar (1971) mental rotation is an example of transforming a mental representation in a continuous process Mental Representations • Mental Representations can be transformed – Shepard & Metzlar (1971) mental rotation is an example of transforming a mental representation in a continuous process – The time it takes to respond is linearly determined by the number of degrees one has to rotate – Somehow the brain must perform a set of operations on these representations - where? how? Mental Representations • Mental Representations can be transformed into abstract information representations – Posner letter matching task – Are these letters from the same category (vowels or consonants) or are they different? Mental Representations • Mental Representations can be transformed into abstract information representations – Posner letter matching task – Are these letters from the same category (vowels or consonants) or are they different? – Are they physically the same or are they the same in an abstract way - they are in the same category? A A A a SAME A U S C A S DIFFERENT Mental Representations • Mental Representations can be transformed into abstract information representations – Posner letter matching task – Participants are fastest when the response doesn’t require transforming the representation from a direct manifestation of the stimulus into something more abstract Mental Representations • Mental Representations can interfere – Stroop task: name the colour in which the word is printed (I.e. don’t read the word, just say the colour Mental Representations • Mental Representations can interfere – Stroop task: name the colour in which the word is printed (I.e. don’t read the word, just say the colour RED Mental Representations • Mental Representations can interfere – Stroop task: name the colour in which the word is printed (I.e. don’t read the word, just say the colour BLUE Mental Representations • Mental Representations can interfere – Stroop task: name the colour in which the word is printed (I.e. don’t read the word, just say the colour GREEN Mental Representations • Mental Representations can interfere – Stroop task: name the colour in which the word is printed (I.e. don’t read the word, just say the colour RED Mental Representations • Mental Representations can interfere – Stroop task: name the colour in which the word is printed (I.e. don’t read the word, just say the colour BLUE Mental Representations • Mental Representations can interfere – Stroop task: name the colour in which the word is printed (I.e. don’t read the word, just say the colour GREEN Mental Representations • Mental Representations can interfere – Stroop task: name the colour in which the word is printed (I.e. don’t read the word, just say the colour – The mental representation of the colour and the representation of the text are incongruent and interfere – one representation must be selected and the other suppressed – This is one conceptualization of attention VISION SCIENCE Visual Pathways • Themes to notice: – Contralateral nature of visual system – Information is organized: • According to spatial location • According to features and kinds of information Visual Pathways • Image is focused on the retina • Fovea is the centre of visual field – highest acuity • Peripheral retina receives periphery of visual field – lower acuity – sensitive under low light Visual Pathways • Retina has distinct layers Visual Pathways • Retina has distinct layers • Photoreceptors – Rods and cones respond to different wavelengths Visual Pathways • Retina has distinct layers • Amacrine and bipolar cells perform “early” processing – converging / diverging input from receptors – lateral inhibition leads to centre/surround receptive fields - first step in shaping “tuning properties” of higherlevel neurons Visual Pathways • Retina has distinct layers – signals converge onto ganglion cells which send action potentials to the Lateral Geniculate Nucleus (LGN) – two kinds of ganglion cells: Magnocellular and Parvocellular • visual information is already being shunted through functionally distinct pathways as it is sent by ganglion cells Visual Pathways • visual hemifields project contralaterally – exception: bilateral representation of fovea! • Optic nerve splits at optic chiasm • about 90 % of fibers project to cortex via LGN • about 10 % project through supperior colliculus and pulvinar – but that’s still a lot of fibers! Note: this will be important when we talk about visuospatial attention Visual Pathways • Lateral Geniculate Nucleus maintains segregation: – of M and P cells (mango and parvo) – of left and right eyes P cells project to layers 3 - 6 M cells project to layers 1 and 2 Visual Pathways • Primary visual cortex receives input from LGN – also known as “striate” because it appears striped when labeled with some dyes – also known as V1 – also known as Brodmann Area 17 Visual Pathways • Primary cortex maintains distinct pathways – functional segregation • M and P pathways synapse in different layers W. W. Norton The Role of “Extrastriate” Areas • Consider two plausible models: 1. System is hierarchical: – each area performs some elaboration on the input it is given and then passes on that elaboration as input to the next “higher” area 2. System is analytic and parallel: – different areas elaborate on different features of the input The Role of “Extrastriate” Areas • Different visual cortex regions contain cells with different tuning properties The Role of “Extrastriate” Areas • Functional imaging (PET) investigations of motion and colour selective visual cortical areas • Zeki et al. • Subtractive Logic – stimulus alternates between two scenes that differ only in the feature of interest (i.e. colour, motion, etc.) The Role of “Extrastriate” Areas • Identifying colour sensitive regions Subtract Voxel intensities during these scans… …from voxel intensities during these scans …etc. Time -> The Role of “Extrastriate” Areas • result – voxels are identified that are preferentially selective for colour – these tend to cluster in anterior/inferior occipital lobe The Role of “Extrastriate” Areas • similar logic was used to find motion-selective areas Subtract Voxel intensities during these scans… MOVING …from voxel intensities during these scans STATIONARY MOVING Time -> STATIONARY …etc. The Role of “Extrastriate” Areas • result – voxels are identified that are preferentially selective for motion – these tend to cluster in superior/dorsal occipital lobe near TemporoParietal Junction – Akin to Human V5 The Role of “Extrastriate” Areas • Electrical response (EEG) to direction reversals of moving dots generated in (or near) V5 • This activity is absent when dots are isoluminant with background • Tata, Mason & Sutherland (2007) The Role of “Extrastriate” Areas • Thus PET studies doubly-dissociate colour and motion sensitive regions The Role of “Extrastriate” Areas • V4 and V5 are doubly-dissociated in lesion literature: The Role of “Extrastriate” Areas • V4 and V5 are doubly-dissociated in lesion literature: – achromatopsia (color blindness): • there are many forms of color blindness • cortical achromatopsia arises from lesions in the area of V4 • singly dissociable from motion perception deficit - patients with V4 lesions have other visual problems, but motion perception is substantially spared The Role of “Extrastriate” Areas • V4 and V5 are doubly-dissociated in lesion literature: – akinetopsia (motion blindness): • bilateral lesions to area V5 (extremely rare) • severe impairment in judging direction and velocity of motion - especially with fast-moving stimuli • visual world appeared to progress in still frames • similar effects occur when M-cell layers in LGN are lesioned in monkeys How does the visual system represent visual information? How does the visual system represent features of scenes? • Vision is analytical - the system breaks down the scene into distinct kinds of features and represents them in functionally segregated pathways • but… • the spike timing matters too! Visual Neuron Responses • Unit recordings in LGN reveal a centre/surround receptive field • many arrangements exist, but the “classical” RF has an excitatory centre and an inhibitory surround • these receptive fields tend to be circular - they are not orientation specific How could the outputs of such cells be transformed into a cell with orientation specificity? Visual Neuron Responses • LGN cells converge on “simple” cells in V1 imparting orientation (and location) specificity Visual Neuron Responses • LGN cells converge on simple cells in V1 imparting orientation specificity • Thus we begin to see how a simple representation - the orientation of a line in the visual scene - can be maintained in the visual system – increase in spike rate of specific neurons indicates presence of a line with a specific orientation at a specific location on the retina – Why should this matter? Visual Neuron Responses • Edges are important because they are the boundaries between objects and the background or objects and other objects Visual Neuron Responses • This conceptualization of the visual system was “static” - it did not take into account the possibility that visual cells might change their response selectivity over time – Logic went like this: if the cell is firing, its preferred line/edge must be present and… – if the preferred line/edge is present, the cell must be firing • We will encounter examples in which neither of these are true! • Representing boundaries must be more complicated than simple edge detection! Visual Neuron Responses • Boundaries between objects can be defined by color rather than brightness Visual Neuron Responses • Boundaries between objects can be defined by texture Visual Neuron Responses • Boundaries between objects can be defined by motion and depth cues