Visual awareness Things to do: * Create slides * Look up additional sources * Read through recent papers on visual awareness * Insert slides on the following: speculate about, and elicit discussion from the class regarding the possible functions of visual awareness. Why couldn't we have operated in a 'blindsight' kind of mode? * Be sure to bring water I. Prior lectures (1 slide) A. Mostly focused on unconscious processes B. Unconscious processes explain why vision is deceptively hard II. Visual awareness, visual experience, visual consciousness (5 slides) A. Hard to define (it feels like...) B. Seems to happen higher up in the neuron chain (not at the photoreceptor level) 1. Filling in of blind spots - does not rely directly on photoreceptor input (show 13.0.1) 2. Filling in of scotomas (show 13.0.2) C. Philosophical questions 1. What determines which processes are conscious ones? 2. At what level do these visual experiences arise? 3. How does these visual experiences arise? 4. What is the relation between consciousness and which neurons are firing in the brain (and how)? D. No good answers to the above questions 1. Are the answers at the algorithmic level or neurophysiological implementation level? 2. Could we create consciousness by connecting millions of microprocessors which simulated the input/output relationship behavior of every neuron? (are we simply carbon machines?) 3. Explanatory gap - gap between visual experiences (or, experiences in general) and how they arise from physical events in the brain 4. So far, no good contenders to fill in the explanatory gap 5. Hubel and Wiesel experiments and other physiological experiments probably won't explain consciousness because subjects are typically unconscious when their neurons are being recorded; some work has made progress to overcome this (Maunsell and Newsome, 1987). III. Philosophical foundations (10 slides) A. Mind-body problem 1. Concerns relation between mental events (e.g. perceptions, pains, hopes, desires, and beliefs) and physical events in the brain (brain activity) 2. Dualism a. Mind and body are two different kinds of entities B. The problem of other minds 1. Criteria for consciousness 2. Inverted spectrum argument 3. Phenomenological criteria 4. Behaviorial criteria (Turing's test) a. Eliza program 5. Physiological criteria a. Terry Schiavo (?) 6. Correlational versus causal theories IV. Neuropsychology of visual awareness (10 slides) A. Definition of vision 1. "The process of acquiring knowledge about environmental objects and events by extracting information from the light they emit or reflect" 2. This definition does not mention visual awareness 3. Many vision processes occur without being aware of them (e.g. when an object comes toward us and we turn our attention to it) 4. Is it possible that a full perceptual analysis can occur without visual awareness? B. Split-brain patients 1. Corpus collosum (show 13.2.1) a. Large tract of fibers that connect the two cerebral hemispheres b. Gustav Fechner (1860) conjectured that this is necessary for the unity of consciousness c. Thought experiment: if we were to sever the corpus collosum, would we then get a person with two different consciences? d. 1940's (look up images): first surgeries to sever the corpus callosum to reduce epileptic seizures 1) Seizures would begin in one hemisphere and then move to other hemisphere 2) Neurosurgeons reasoned that cutting corpus callosum would drastically reduce the frequency and severity of seizures (which was true in many individuals) 3) At first, it seemed that there was little effect on consciousness (Karl Lashley joked that perhaps the function of the corpus callosum was simply to hold the two hemispheres together) e. Roger Sperry (1961) and Michael Gazzaniga (1970) on patient N.G. 1) NG fixated on a point in the middle of a screen 2) A picture of a cup was flashed to the right of the dot 3) NG was queried and replied that she saw a cup 4) Then, a picture of a spoon was flashed to the left of the dot 5) NG was queried and replied that she saw nothing 6) NG was then asked to reach under the screen with her left hand and pick out the object that had appeared in her left visual field; she did this without seeing the objects 7) NG reached under and felt each object; she finally held up the spoon 8) When asked what she was holding, she replied, "a pencil" f. Processing of information by NG (show figure 13.2.2) 1) Images in left visual field (LVF) are processed in the right hemisphere (RH) and vice versa 2) Since NG appeared to perceive only the cup, is only the LH conscious? a) Possibly, but not necessarily b) Speech centers are normally located in the LH c) In normal brains, images processed in the RH cross via the corpus callosum to the LH; NG could not do this because of the sever 3) Does NG's inability to name the spoon (and inability to say that she saw anything at all) mean that RH is not conscious or does it mean that RH has its own consciousness without expressive linguistic capabilities? a) We come back to the question of what evidence will satisfy us to believe that someone or something is conscious? (similarity to me evidence) b) Most likely explanation: each hemisphere is aware of the object but only LH can talk about it C. Blindsight 1. Ability of certain patients to do better than chance on visual tasks but report that they cannot see a. First documented case: Weiskrantz (1980) and Weiskrantz, Sanders, and Marshall (1974) b. Discovery resulted after trying to reconcile monkey's discriminative performance after their striate cortex was removed c. Similar operation was performed on humans for medical reasons 2. The case of D.B. a. Had severe migraine problems due to enlarged blood vessels in the right visual cortex b. The part of the brain containing the blood vessels was removed c. Migraines stopped and quality of life was much improved d. DB was blind (as expected) in the LVF; he was tested by recording whether he saw a point source light in various regions of his visual field (show 13.2.3) 3. Accurate guessing without visual experience a. A point light source was presented along the horizontal midline of DB's blind or sighted fields b. DB was asked to guess where the light source was by pointing to it (whether he saw it or not) (show 13.2.4) c. DB had remarkable success given that he was "guessing" when the light was in the LVF d. In another experiment, DB was asked to make disciminations between X and O, horizontal and vertical, and diagonal versus vertical lines without consciously seeing them 1) Larger stimuli and longer duration improved performance 2) DB was concientious of reporting when he visually saw something 3) Otherwise, DB was merely guessing e. How was DB's performance possible? 4. The two visual systems hypothesis a. In DB's case, the primary pathway to visual cortex was removed b. However, a secondary visual pathway to the superior colliculus in the thalamus was still intact c. Perhaps cortical system is responsible for awareness and colliculus system performed significant nonconscious functions d. The pathway through the superior colliculus is smaller than the cortical system (and hence has lower spatial resolution than the cortical system) which explains why performance increased when the stimulus was larger e. Cowey and Stoerig (1995) experiments on monkeys (look up this paper and contrast with Weiskrantz' work, see above) 1) They removed area V1 of one cerebral hemisphere from three monkeys 2) They verified that they had residual visual abilities in the opposite visual half-field 3) They then trained the monkeys in their intact visual field to discriminate between real visual events and blanks (no stimulus) 4) They then tested whether the monkeys would respond to a real stimulus in their impaired hemisphere; they responded as if it was a blank 5) This implies that monkeys do not have visual experiences in the impaired hemifield 5. Methodological challenges a. DB's tests were less than perfectly executed; his eye movements were not tracked -> the stimulus may have landed in an area that he could perfectly see b. The light from the stimulus presented in the blind visual field could have scattered in his eye and into the sighted RVF -> later experiments showed that this is false for at least some aspects of blindsighted performance c. The patient CLT (Fendrich, Wessinger, and Gazzaniga, 1992) 1) Patient suffered a stroke in right occipital region 2) MRI showed extensive damage to visual cortex with islands of intact tissue 3) Superior colliculus was entirely unaffected because it is supplied by a different blood stream 4) Eye movement was precisely tracked and stimuli was presented to precise locations 5) Experimenters tested residual visual function throughout the retina and found it to be present in certain, localized areas (performance was at chance for most areas of the LVF except for small, localizable areas) 6) CLT reported no visual experience from the small localized areas 7) This challenges the theory that the unconscious superior colliculus is mediating blindsight 8) Need to reconcile Cowley and Stoerig's (1995) results; perhaps monkey's mechanisms are different from humans 9) In humans, there is evidence that the collicular pathway is involved in blindsight in terms of eye movements: eye movements of blindsighted patients can be influenced by stimulation in the blind field (Rafal, Smith, Krantz, Cohen, and Brennen, 1990) 10) Possibly other connections other than the collicular pathway is involved? (LGN project to V4 and MT) 6. Blindsight summary a. Patient can perform better than chance on discrimination tasks with small number of alternatives and is forced to guess b. However, patient cannot "see" based on the bottom-up processing of sensory information c. The residual visual function requires top-down testing of a specific hypothesis, provided by the experimenters; the patient cannot formulate the hypotheses on their own d. Hence, blightsight is not helpful since the patient cannot perform spontaneous intentional actions D. Unconscious processing in Neglect and Balint's Syndrome E. Unconscious face recognition in prosopagnosia V. Visual awareness in normal observers (10 slides) A. Can we obtain evidence of dissociation between visual processing and visual awareness? B. Perceptual defense C. Subliminal perception 1. Ability to register and process information that was presented below the threshold of awareness a. Blindsight is an example b. Subliminal experimentation scheme 1) Direct task a) E.g. simple detection task where subjects indicate whether they saw something b) When subject's performance is at chance, it is assumed that the subject is not visually aware of the stimulus 2) Indirect task: subject asked to perform task that uses information from the stimulus, even though the subject was not aware of the stimulus 2. Anthony Mercel's experiments (1983) (figure 13.3.1) a. Used yes/no detection performance as direct measure of conscious experience 1) Flashed color words (RED, BLUE, GREEN, YELLOW) or blank slide, followed by a pattern mask 2) Subjects gave yes/no response as to whether they saw a word 3) Duration of the word was adjusted to get 60% detection rate (between 30 to 80 ms) b. Used performance on Stroop color-naming task as the indirect measure of unconscious processing 1) Suprathreshold trial a) Subjects see color word (or blank) and color patch together for 400 msec. b) Pattern mask then covers color word c) Subjects must name as quickly as possible the color patch, regardless of the word that appears d) Sometimes, the word and color patch were consistent; other times, not e) Subjects consciously perceive the word f) Both interference and facilitation are present 2) Subthreshold a) Subjects see color word (or blank) and color patch together for recorded duration above that yielded slightly above chance detection of word b) The idea is that the subject is not consciously aware of the word c) The pattern mask is then presented (covering the word) d) Subjects showed significant interference and facilitation e) Therefore, words were registered c. Did subjects actually not experience the word? Or simply, were they reticent to say they had? 3. Objective versus subjective thresholds of awareness a. Cheesman and Merikle (1984) experiments 1) Subjects were too conservative in reporting that they had not seen the words 2) Best way to find the threshold of conscious perception is to have subjects make a forced-choice discrimination of the four color words: objective threshold of awareness a) color was is shown b) then pattern mask c) threshold is chosen so that subject performance at chance (25%) 3) Marcel experiment was repeated, but with new threshold 4) No Stroop effects were observed 5) Marcel's threshold (did you see anything or not?) is subjective threshold of awareness 6) Properties of objective threshold a) Near threshold, subjects report that they are guessing randomly, even though they are performing above chance b) This implies that nonconscious processing is included as awareness c) Should nonconscious processing be included as awareness? Compare this with patient D.B. (blindsight) b. Exhaustiveness 1) The measure of where the threshold of awareness lies should exhaust the contents of consciousness 2) This is ground of criticism of Marcel's experiments c. Exclusiveness 1) The measure of where the threshold of awareness lies should only measure aspects of conscious experience 2) This is grounds of criticism of Cheesman and Merikle experiments d. Gray area 1) Both thresholds provide bounds of consciousness 4. Functional correlates of consciousness D. Inattentional blindsight VI. Theories of consciousness (10 slides) A. VII. New results in visual awareness (5 slides) Sources: Victor Lamme. "Why visual attention and awareness are different," TRENDS IN COGNITIVE SCIENCES, 7 (1): 12-18 JAN 2003. Eliza: What makes you think you would like to kill everyone? There's a little picture of ED MCMAHON doing BAD THINGS to JOAN RIVERS in a $200,000 MALIBU BEACH HOUSE!! Is it because of some problems at home that you are going through all this? WHOA!! I'm having a RELIGIOUS EXPERIENCE right NOW!! Why do you say you are having a religious experience right now?