HST 722 Brain Mechanisms for Hearing and Speech. Neural Bases of Speech Motor Control. Frank Guenther

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1 HST 722 Brain Mechanisms for Hearing and Speech Neural Bases of Speech Motor Control Frank Guenther HST 722 Speech Motor Control 1

2 Early Studies Penfield and Colleagues Wilder Penfield performed neurosurgery at the Montreal Neurological Institute in the 1940s and 1950 s. Focal cerebral epilepsy Injury to a brain area (e.g., stroke) can lead to abnormal activity in the immediate or surrounding gray matter. This abnormal activity may occur years after injury. A damaged cortical area that acts as a self-discharging electrochemical unit is called an epileptogenic focus. From time to time, explosive discharges are released by this area; these discharges are called epileptic fits, characterized by loss of consciousness, contraction of many or all muscles, groaning, and frothing at the mouth. In severe cases, the epileptogenic focus is removed via surgery. Penfield and colleagues used electrical stimulation to carefully map the cortical areas before performing such operations to try to minimize damage. HST 722 Speech Motor Control 2

Cortical Stimulation Prior to Epilepsy Surgery FIG. VII-l. Layout of operating room. The surgeon is filling in a sketch of the brain. In the background Dr.

3 Cortical Stimulation Prior to Epilepsy Surgery FIG. VII-l. Layout of operating room. The surgeon is filling in a sketch of the brain. In the background Dr. Jasper is seen recording an electrocorticogram behind the glass partition of the observing stand, while his associate, Dr. Guy Courtois, watches. The patient is seen fully conscious and comfortable, thanks to local analgesia, with the anesthetist, Dr. Andre Pasquet, who is acting as observer, beside him. Beyond the observer is the square window through which the photographs of the brain are taken by means of the mirror placed above the table. HST 722 Speech Motor Control 3

Areas of the Brain Where Stimulation Has a Noticeable Effect FIG. 111-1.

4 Areas of the Brain Where Stimulation Has a Noticeable Effect FIG Photograph of left hemisphere of the human brain. HST 722 Speech Motor Control 4

5 Auditory cortex stimulation Patient hears ringing, humming, clicking, rushing, chirping, buzzing, knocking, or rumbling. He is never under the impression that he has heard words, nor music, nor anything that represents a memory. Somatosensory cortex stimulation Patient reports a sensation of tingling or numbness, or of movement in some particular part. But he is never under the impression that he has touched an external object. He considers it an artifact, not an ordinary sensation. Visual cortex stimulation what the patients see is much more elementary than things seen in ordinary life [they see] flickering lights, colors, stars, wheels, blue-green and red colored disks a long white mark, shadow moving, black wheel, etc. Motor cortex stimulation the patient may be astonished to discover that he is moving his arm or leg. He may be surprised to hear himself vocalizing, but he never has the impression that he has willed himself to do these things. HST 722 Speech Motor Control 5

6 Effects of Direct Cortical Stimulation on Speech FIG. VIII-1. Vocalization, which is a sustained or interrupted vowel cry, has been produced by stimulation at these points in the left hemisphere. HST 722 Speech Motor Control 6

7 FIG. VIII-2. Vocalization has occurred during stimulation at these points in the right hemisphere. HST 722 Speech Motor Control 7

8 FIG. VI11-3. Electrical interference has produced interference with speech at these points in the left hemisphere. When the electric current has been applied to other areas, such as the frontal pole or the occipital lobe, no difficulty in speech has been noted. The types of disturbance are given in subsequent figures. HST 722 Speech Motor Control 8

9 Motor Cortex and Movement Control Motor cortex used to refer to either primary motor cortex (Brodmann s Area 4, M1) or both Area 4 and Area 6 (supplementary motor area, premotor cortex). Area 4 stimulation produces movements with lowest currents of any area in the brain. Some commands to the fingers, hand, and speech articulators are mediated by direct connections from motor cortex (BA 4) to the spinal cord or brain stem motoneurons. However, most motor output commands involve subcortical structures such as the red nucleus and cerebellum. HST 722 Speech Motor Control 9

The Motor Homunculus In humans, the speech articulators and fingers have very large primary motor cortex representations The somatosensory and motor homunculi are

10 The Motor Homunculus In humans, the speech articulators and fingers have very large primary motor cortex representations The somatosensory and motor homunculi are approximately aligned with each other Corresponding parts of primary somatosensory cortex and primary motor cortex are interconnected HST 722 Speech Motor Control 10

11 Figure 38.3 Sites controlling an individual muscle are not located together but are distributed over a wide area of motor cortex. Intracortical microstimulation was used to identify sites in monkey primary motor cortex at which low-threshold stimulation evoked electromyographic activity (indicating monosynaptic connections) in a shoulder abduction muscle (middle head of deltoid muscle) and a wrist extensor muscle (extensor carpi radialis; ECR). Topographic maps of the identified sites reveal overlap between shoulder and wrist representations. The maps were constructed based on the inverse of the threshold (1/threshold) in microamperes, with the peaks representing approximately 1/3 microamps and the valleys 1/40 microamps (From Humphrey DR, Tanji J In: DR Humphrey, HJ Freund (eds.). Motor Control: Concepts and Issues, pp New York: Wiley.) PNS p. 759 HST 722 Speech Motor Control 11

12 Basal Ganglia and Cerebellum in Movement Control HST 722 Speech Motor Control 12

13 The Basal Ganglia HST 722 Speech Motor Control 13

14 Basal Ganglia-Thalamo-Cortical Loop HST 722 Speech Motor Control 14

to cause a planned movement to be produced, and by inhibiting

15 The basal ganglia are important for movement selection and initiation, perhaps by providing a gating signal back to cortex to cause a planned movement to be produced, and by inhibiting the gating signals for unwanted movements. HST 722 Speech Motor Control 15

16 Parkinson s Disease Parkinson s disease is the result of damage to the substantia nigra pars compacta, which supplies the neurotransmitter dopamine to the input part of the basal ganglia (the striatum, composed of the caudate nucleus and the putamen). Symptoms include: Rigidity Tremor Lessened muscular power Slowness of movement with movement initiation difficulty (bradykinesia) Abnormally small movements (hypometria), including micrographia Fast gait with small steps [See INS Dictionary of Neuropsychology] HST 722 Speech Motor Control 16

17 Huntington s Disease Another disease of the basal ganglia, Huntington s disease (formerly called Huntington s chorea), has symptoms that are different from, and in some ways opposite to, those of Parkinson s disease: Patients exhibit excess, or unsuppressed, movements, referred to as chorea (from the Greek word for dance ). Patients later suffer from dementia and other psychiatric disorders. Chorea can be preceded by a period of depression (up to 10 years before onset of movement disorder). Huntington s disease is associated with severe atrophy in the striatum, particularly the head of the caudate nucleus. It has been suggested that the neuropsychological aspects of Huntington s disease might be the result of compromised prefrontal cortex operation (the frontal lobe is heavily interconnected with the striatum, particularly the head of the caudate nucleus). [See INS Dictionary of Neuropsychology] HST 722 Speech Motor Control 17

18 Direct and Indirect Pathways of the Basal Ganglia Pathways through the basal ganglia are often broken into a direct (striatum->gpi) and indirect (striatum->gpe->stn->gpi) pathway. The output of the basal ganglia (Gpi) is inhibitory, as are the direct projections from striatum to GPi. Excess activation in the direct pathway facilitates movement; excess activation in the indirect pathway inhibits movement. HST 722 Speech Motor Control 18

Difference Between Parkinson s and Huntington s Diseases Parkinson s disease can be characterized as a shift in the balance of activity toward the indirect pathway, thus inhibiting movement.

19 Difference Between Parkinson s and Huntington s Diseases Parkinson s disease can be characterized as a shift in the balance of activity toward the indirect pathway, thus inhibiting movement. This occurs due to differential effects of dopamine on the direct and indirect pathways. Thus, lesions or disruptive stimulation (deep brain stimulation, DBS) of the subthalamic nucleus (STN) or GPi can actually help relieve Parkinson s symptoms. Huntington s disease can be characterized as a shift in the balance of activity toward the direct pathway, thus causing excess movement. Pink pathways are excitatory; gray are inhibitory HST 722 Speech Motor Control 19

20 Cerebellum The cerebellar cortex has a very regular and well-studied structure: HST 722 Speech Motor Control 20

21 Ataxia Greek meaning: without order. Ataxia is characterized by abnormal movement, including errors in range, direction, timing, and force ( coordination ) of motor activity. Components of ataxia include trouble decomposing movements into component parts, incorrect movement distance (dysmetria), and tremor. Ataxia is usually the result of damage to the cerebellum. Ataxic dysarthria slurred, slow, and uncoordinated speech caused by cerebellum damage (patients often appear to be inebriated). [See INS Dictionary of Neuropsychology] HST 722 Speech Motor Control 21

22 The Cerebellum as a Side Loop Figure 42-6 Synaptic organization of the basic cerebellar circuit module. Mossy and climbing fibers convey output from the cerebellum via a main excitatory loop through the deep nuclei. This loop is modulated by an inhibitory side loop passing through the cerebellar cortex. This figure shows the excitatory (+) and inhibitory (-) connections among the cell types. HST 722 Speech Motor Control 22

23 Learning in the cerebellar side loop results in the tuning of motor commands: Cerebellar cortex Tunable side-loop Input from cortex Motor command This accounts for the role of the cerebellum in maintaining smooth, wellcoordinated movements. HST 722 Speech Motor Control 23

showing the arm and leg representations.

M1 =primary motor cortex; PMd=dorsal premotor area; PMv=ventral premotor area; PS=precentral

24 Premotor Cortical Areas PNS, p. 757 <= Somatotopic organization of the medial and lateral motor cortex in the monkey, showing the arm and leg representations. (ArSi, arcuate sulcus, inferior limb; ArSs=arcuate sulcus, superior limb; CS=central sulcus; M1 =primary motor cortex; PMd=dorsal premotor area; PMv=ventral premotor area; PS=precentral sulcus; SGm=superior frontal gyrus, medial wall; SMA=supplementary motor area; pre-sma = presupplementary motor area; SPcS=superior precentral sulcus.) (From Dum and Strick 1996.) HST 722 Speech Motor Control 24

25 PNS p. 760 HST 722 Speech Motor Control 25

26 Onset times from Kalaska & Crammond (1992): HST 722 Speech Motor Control 26

27 The medial premotor area (medial BA 6), is called the supplementary motor area (SMA) and is important for the planning of internally generated movements and mental rehearsal: HST 722 Speech Motor Control 27

28 Brodmann s areas 8 (frontal eye fields) and 44/45 (Broca s area) can be considered to be premotor areas specialized for eye movements (area 8) and speech (areas 44/45). 8 44/45 HST 722 Speech Motor Control 28

29 Global Organization of Motor Circuits Rizzolatti et al. (1998): Classical definitions of motor cortex highlight 2 or 3 functionally and cytoarchitectonically distinct areas: primary motor cortex (BA 4), premotor cortex (lateral BA 6), and supplementary motor area (SMA; medial BA 6). More recent studies indicate a more complex organization, with at least seven distinct motor cortical areas in the frontal lobe of the macaque monkey (F1-F7). Rizzolatti et al. (1998) Fig. 1 HST 722 Speech Motor Control 29

30 Global Organization of Motor Circuits (cont d) The motor areas are interconnected with distinct parietal areas to form several somewhat separate motor circuits with different functionalities. Rizzolatti et al. (1998) Fig. 1 HST 722 Speech Motor Control 30

31 Separate circuits for reaching (VIP/F4) and grasping (AIP/F5): HST 722 Speech Motor Control 31

32 PF F5c Circuit Motor properties of F5c neurons same as F5ab neurons, but visual properties differ: F5c neurons will respond when monkey observes another individual performing an action similar to that coded by the neuron (e.g., grasping, placing, or manipulating objects). Mirror neurons PF neurons have similar visual properties. HST 722 Speech Motor Control 32

33 Prefrontal Lobe F6 Circuit Unlike other motor areas, F6 (pre- SMA) has few connections with parietal lobe, instead connecting with frontal lobe areas BA 46 (dorsolateral prefrontal cortex) and the rostral cingulate cortex (BA 24c). Seems to be involved in control of motor processing in the parieto-frontal circuits rather than in sensorimotor transformations. May also play an important role in sequence organization (Tanji and colleagues). HST 722 Speech Motor Control 33

34 Human: Possible Human Monkey Homologues Monkey: Matching colors indicate possible homologues HST 722 Speech Motor Control 34

Making Things Happen 2: Motor Disorders

Making Things Happen 2: Motor Disorders How Your Brain Works Prof. Jan Schnupp wschnupp@cityu.edu.hk HowYourBrainWorks.net On the Menu in This Lecture In the previous lecture we saw how motor cortex and