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                                                                    Exhibit 10.1

                        SIXTH AMENDMENT TO LOAN AGREEMENT

         THIS AMENDMENT is made as of October 8, 2004 by ASPECT MEDICAL SYSTEMS,
INC. (the "Borrower") and FLEET NATIONAL BANK (the "Lender").

                                    RECITALS

         A. The Lender and the Borrower are parties to a letter agreement dated
as of May 16, 2001, as amended (as amended, the "Loan Agreement"). Capitalized
terms used herein without definition have the meanings assigned to them in the
Loan Agreement.

         B. The Borrower has requested that the Bank permit a renewal of a
certain standby Letter of Credit issued for the account of the Borrower.

         NOW THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, the parties hereto agree as
follows:

I.       AMENDMENT TO LOAN AGREEMENT. Notwithstanding the provisions of Section
1.6(b)(i)(A) of the Loan Agreement which prohibit the expiration of any Letter
of Credit later than 90 days after the Expiration Date, the Lender and the
Borrower hereby agree that that certain Letter of Credit issued for the benefit
of Newton Technology Park, LLC in the face amount of $80,000 (the "Newton L/C")
shall be permitted to automatically renew as of December 31, 2004 (the "Renewal
Date") for a period of one year, with the result that the expiration date of the
Newton L/C shall be December 31, 2005.

II.      NO FURTHER AMENDMENTS.

         Except as specifically amended hereby, the Loan Agreement and the
Pledge Agreement shall remain unmodified and in full force and effect and are
hereby ratified and affirmed in all respects, and the indebtedness of the
Borrower to the Lender evidenced thereby and by the Revolving Note is hereby
reaffirmed in all respects. On and after the date hereof, each reference in the
Loan Agreement to "this letter agreement", "hereunder", "hereof", or words of
like import referring to the Loan Agreement, shall mean and be a reference to
the Loan Agreement as amended by this Amendment, and each reference in Pledge
Agreement to the Loan Agreement, "thereunder", "thereof", or words of like
import referring to the Loan Agreement shall mean a reference to the Loan
Agreement as amended by this Amendment.

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III.     CONFIRMATION OF SECURITY.

         The Pledge Agreement shall remain in full force and effect and is
hereby ratified and affirmed in all material respects. The Borrower hereby
acknowledges and agrees that the "Obligations" secured by, and entitled to, the
benefits of the Pledge Agreement include, without limitation, the Revolving
Note.

IV.      MISCELLANEOUS.

         A. As provided in the Loan Agreement, the Borrower agrees to reimburse
the Lender upon demand for all out-of-pocket costs, charges, liabilities, taxes
and expenses of the Lender (including reasonable fees and disbursements of
counsel to the Lender) in connection with the (a) preparation, negotiation,
interpretation, execution and delivery of this Amendment and any other
agreements, instruments and documents executed pursuant or relating hereto, and
(b) any enforcement hereof.

         B. This Amendment shall be governed by and construed in accordance with
the laws of the Commonwealth of Massachusetts.

         D. This Amendment may be executed by the parties hereto in several
counterparts hereof and by the different parties hereto on separate counterparts
hereof, all of which counterparts shall together constitute one and the same
agreement.

            **THE BALANCE OF THIS PAGE IS LEFT BLANK INTENTIONALLY**

                                      -2-

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         IN WITNESS WHEREOF, the Lender and the Borrower have caused this
Amendment to be duly executed as a sealed instrument by their duly authorized
representatives, all as of the day and year first above written.

                                            ASPECT MEDICAL SYSTEMS, INC.

                                            By:    /s/ J. Neal Armstrong
                                                --------------------------------
                                                Title: CFO

                                            FLEET NATIONAL BANK

                                            By:    /s/ Peter G. McCarthy
                                                --------------------------------
                                                Title: CPO/SVP