3 An edit by another borked up the formatting of the citation, which is, of course, the whole point of the answer. *AAaarrgghh!*
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I'd use the form for symposia, adjusted slightly to reflect that it's a keynote speech with only one speaker.

For the APA style, see the second block in this blog entry:

Symposia

A symposium is a bit like a live-action edited book: Several authors come together under the leadership of the chair to pool their knowledge about a topic.

Krinsky-McHale, S. J., Zigman, W. B., & Silverman, W. (2012, August).
       Are neuropsychiatric symptoms markers of prodromal Alzheimer’s disease in adults with
       Down syndrome? In W. B. Zigman (Chair), Krinsky-McHale, S. J., Zigman, W. B., & Silverman, W. (2012, August). Are neuropsychiatric symptoms markers of prodromal Alzheimer’s disease in adults with Down syndrome? In W. B. Zigman (Chair), Predictors of mild cognitive impairment, dementia, and
       and mortality in adults with Down syndrome. Symposium conducted at the meeting of the American Psychological Association, Orlando, FL.
Symposium conducted at the meeting of the
       American Psychological Association, Orlando, FL.

I'd use the form for symposia, adjusted slightly to reflect that it's a keynote speech with only one speaker.

For the APA style, see the second block in this blog entry:

Symposia

A symposium is a bit like a live-action edited book: Several authors come together under the leadership of the chair to pool their knowledge about a topic.

Krinsky-McHale, S. J., Zigman, W. B., & Silverman, W. (2012, August). Are neuropsychiatric symptoms markers of prodromal Alzheimer’s disease in adults with Down syndrome? In W. B. Zigman (Chair), Predictors of mild cognitive impairment, dementia, and mortality in adults with Down syndrome. Symposium conducted at the meeting of the American Psychological Association, Orlando, FL.

I'd use the form for symposia, adjusted slightly to reflect that it's a keynote speech with only one speaker.

For the APA style, see the second block in this blog entry:

Symposia

A symposium is a bit like a live-action edited book: Several authors come together under the leadership of the chair to pool their knowledge about a topic.

Krinsky-McHale, S. J., Zigman, W. B., & Silverman, W. (2012, August).
       Are neuropsychiatric symptoms markers of prodromal Alzheimer’s disease in adults with
       Down syndrome? In W. B. Zigman (Chair), Predictors of mild cognitive impairment, dementia,
       and mortality in adults with Down syndrome.
Symposium conducted at the meeting of the
       American Psychological Association, Orlando, FL.

2 add cited passage
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I'd use the form for symposia, adjusted slightly to reflect that it's a keynote speech with only one speaker. For

For the APA style, see the second block in this blog entry: http://blog.apastyle.org/apastyle/2012/08/how-to-cite-materials-from-meetings-and-symposia.htmlthis blog entry  :

Symposia

A symposium is a bit like a live-action edited book: Several authors come together under the leadership of the chair to pool their knowledge about a topic.

Krinsky-McHale, S. J., Zigman, W. B., & Silverman, W. (2012, August). Are neuropsychiatric symptoms markers of prodromal Alzheimer’s disease in adults with Down syndrome? In W. B. Zigman (Chair), Predictors of mild cognitive impairment, dementia, and mortality in adults with Down syndrome. Symposium conducted at the meeting of the American Psychological Association, Orlando, FL.

I'd use the form for symposia, adjusted slightly to reflect that it's a keynote speech with only one speaker. For the APA style, see the second block in this blog entry: http://blog.apastyle.org/apastyle/2012/08/how-to-cite-materials-from-meetings-and-symposia.html  

I'd use the form for symposia, adjusted slightly to reflect that it's a keynote speech with only one speaker.

For the APA style, see the second block in this blog entry:

Symposia

A symposium is a bit like a live-action edited book: Several authors come together under the leadership of the chair to pool their knowledge about a topic.

Krinsky-McHale, S. J., Zigman, W. B., & Silverman, W. (2012, August). Are neuropsychiatric symptoms markers of prodromal Alzheimer’s disease in adults with Down syndrome? In W. B. Zigman (Chair), Predictors of mild cognitive impairment, dementia, and mortality in adults with Down syndrome. Symposium conducted at the meeting of the American Psychological Association, Orlando, FL.

1
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I'd use the form for symposia, adjusted slightly to reflect that it's a keynote speech with only one speaker. For the APA style, see the second block in this blog entry: http://blog.apastyle.org/apastyle/2012/08/how-to-cite-materials-from-meetings-and-symposia.html